As the United Nations hosts scores of world leaders at its annual General Assembly this month — and a special summit called by Secretary General Ban Ki-Moon for Sept. 20 — behind-the-scene tensions are high between the world organization and New York City, which has repeatedly warned that the U.N. complex on Manhattan’s East side is dangerously exposed to potential terrorist attacks.

Top city officials, including Mayor Michael Bloomberg and Police Commissioner Ray Kelly, feel frustrated that after years of negotiations and a $1.8 billion U.N. facelift that is now under way, the U.N. is ignoring blunt and dire warnings about the risks faced at the 17-acre complex.

“The city is not satisfied with the U.N.’s response to date,” declared mayoral spokesman Jason Post. “The U.N. has not adopted the city’s security recommendations for the headquarters campus.” Post would not elaborate and declined to answer follow-up questions.

The city’s concerns are major. In some places at the periphery of the U.N. complex, little more than a wheel-barrow full of high explosives could have a disastrous effect. In others, a truck-bomber could drive within a few yards of the complex before setting off a blast that could be as devastating as the 1995 Oklahoma City bombing at the Alfred P. Murrah Federal Building.

Those concerns have special resonance in New York City shortly after the ninth anniversary of 9/11, and while memories are still fresh of the unsuccessful May attempt by Muslim extremist Faisel Shahzad to detonate a car bomb in Times Square on behalf of the Pakistani branch of the Taliban.

They also have resonance for the U.N. itself: In 1995, two years after his arrest, a Sudanese immigrant named Siddig Ibrahim Siddig Ali pleaded guilty to conspiring to drive a bomb under the U.N., allegedly with the help of Sudanese diplomats. The plot was broken up two months after the first World Trade Center bombing.

Now, despite years of planning for the U.N. renovation — and a more than trebling of the original $570 million projected cost — the city strongly feels that neither the U.N. nor the State Department, which manages American host country obligations with the world organization, have committed themselves to anywhere near enough protection for the high-profile international target.

In past months, city officials have expressed the same frustrations in increasingly blunt terms to U.S. and U.N. officials, both in written communications and in face to face meetings. Among other things, Bloomberg has written personally to Secretary of State Hillary Clinton about the issues.

For their part, both U.S. State Department and U.N. security officials say that the security “dialogue” is still a work in progress, and there is no end in sight. But they also indicate that the city’s concerns will not ever be completely met, in part because the U.N. is merely renovating the complex, rather than starting over from scratch.

“This is a collective effort of all three parties,” U.S. Under Secretary of State for Management Patrick Kennedy told Fox News in an interview. “I believe we are all working together to develop a process that will integrate security into the renovations of the United Nations complex that will be satisfactory to everyone.”

Kennedy says he has met twice with New York Police Commissioner Ray Kelly on the issue, though he did not describe the outcome.

Kennedy assured Fox News that “the U.S. Government is looking at the full range of improvements and upgrades” for the complex. Yet he also declared that since “there is no desire on the part of anyone to tear down the entire United Nations complex and start over again,” the issue was discovering “exactly what security upgrades are possible”— and “at a reasonable cost.”

“There is always going to be a debate about what constitutes adequate security,” he said.

The same cost-benefit equation was cited by U.N. Under Secretary General Gregory Starr —formerly the State Department’s own security chief — who also takes much the same conciliatory long-term view. “I think ultimately we are all going to come to a good balanced solution and I think everybody will be reasonably satisfied,” he told Fox News in an interview. “I think most of the discussions have been very fruitful and a lot of the ideas that they have put forward are proper ideas.”

Both Kennedy and Starr declined to discuss specific security details, but they underlined that the danger to human life at the U.N. right now is minimal, as most U.N. employees have been removed from the landmark U.N. secretariat building and other buildings while the two-year renovation takes place.

Many of them, however, remain in less-used buildings or temporary quarters built as transitional space during the renovation.

New York City’s intense concerns, however, are focused on the facility that will remain after the renovation is finished in 2012 — an irregular, roughly rectangular shape bounded by the Franklin Delano Roosevelt highway and the East River, 42nd Street, First Avenue, and 48th Street.

All sides, in the city’s view, are dangerously exposed, and the construction standards of the old landmark buildings leave them far more vulnerable to explosions than modern U.S. government buildings, like the new, fortress-like, 26-story U.S. Mission to the U.N., located across the street from the complex, which opened in August. The new mission building has no windows on the first nine floors as an anti-blast measure, and is built, as are all State Department missions, to classified blast-resistance standards.

But one important part of the U.N. complex, the Conference Building that houses the U.N. Security Council, is vulnerable to attack from an unusual direction: underneath. The Conference Building hangs directly over the FDR, on a concrete platform that is directly exposed to a potential terrorist detonation.

The vulnerability is well-known; when President Obama or enough other high-level dignitaries at the U.N., portions of the FDR are shut down to traffic. But the Conference Building remains a potential target even when the top brass have left town.

Another major vulnerability is an exit ramp from the FDR that curls alongside the south end of the U.N. campus onto 42nd Street, a short distance from the U.N. library building.

Once again, the ramp is shut down to all traffic when high-profile events, like the annual opening session of the U.N. General Assembly, take place.

However, “on a day-to-day basis that ramp is open,” says John Cutter, a retired New York City Police chief of criminal intelligence, who inspected the U.N. campus perimeter with Fox News. “If somebody wanted to make a statement, do damage to the U.N., they could go right up there, set off [a] bomb. You could have a devastating effect.”

Much of that effect, he added, would be psychological, given the U.N. complex’s symbolism.

Cutter believes the exit ramp is "a very hard place to defend.” Yet despite a vulnerability visible to any passer-by, there is not even a blast wall in place to deflect any part of a potential explosion on the ramp — a point that city officials have made strongly.

On the north side of the U.N. complex, the exposure is less dramatic, as much of the surface area is devoted to lawns and gardens. (A large, temporary building, however, is now situated on part of the lawn area.). But here, trucks carrying large loads drive up and under the campus to make deliveries. City officials would like to see the trucks unloaded offsite, and their contents screened.

The U.N. has its official entries, and personal screening for visitors, on the west side of the complex, facing First Avenue. There, the slab-sided Secretariat building is set back considerably from the street — but that distance, Cutter says, is nowhere near as safe as it needs to be.

“You need 1,000 feet of distance” to have true security from a bomb blast, he told Fox News, and the Secretariat building is far closer to the street than that. Moreover, a setback of that magnitude is not even possible in an urban setting, which Cutter says is a “tremendous concern.”

A partial solution would be to put up another blast wall — which would mar the serene image of the U.N. from the side that most pedestrians and tourists see. That idea is unlikely to ever be considered.

Another partial solution would be to make it harder for would-be bombers to get quite as close to the complex, by removing a lane of traffic on the U.N. side of First Avenue and installing anti-truck bollards at the perimeter.

“We are looking at the perimeter,” Starr told Fox News. “We are looking at bollards. We are looking at enhancing the survivability of the buildings.”

The frustrating words there, from the city’s point of view, are “looking at,” as in: not deciding yet. The State Department’s Kennedy takes a similar long view as he describes the “process” that is still ongoing.

“We meet, information is exchanged, briefings are provided by security personnel, by architects and engineers. The questions are asked, and then more information is exchanged. And then there are working groups that take place between different elements of the United States government, different elements of the U.N. staff and their architectural engineers, and then different representatives from different departments from the New York City government.”

“We are working each and every issue and there are different timelines, obviously, for what you might do along one of the four sides.”

“I think the city would be happier if we were moving faster,” Starr admitted, while saying that progress had been made on generating some permits for work on First Avenue. (Coincidentally, those permits seem to have been approved around the time Fox News began requesting interviews on this story.)

Both Starr and Kennedy are convinced that their final security solution will have arrived by the time the renovation is finished. Both officials were leery about discussing costs, but one figure tossed out by Starr -- $50 million — seemed very small in terms of the huge spiral in renovation costs that the U.N. has so far accepted.

The big question, however, is whether the eventual solution will satisfy the City of New York —which is not only protecting the U.N., but the lives of New Yorkers at risk in any attack.

The fact is, both Starr and Kennedy emphasized, that when it gets right down to it, the U.N., is an international organization whose campus is outside the jurisdiction of U.S. law. It doesn’t have to please the city, or even the U.S. government, unless it wants to.

“I don't think it's the city's call to determine what the United Nations is going to do to its facilities,” Starr told Fox News. “Ultimately we have to come up with what we think is the right approach.”

And, whenever that approach ultimately gets decided on, the frustrated City of New York and its first responders — praised by both Starr and Kennedy as the best in the world -- will have to defend it.

Or risk their lives picking up the pieces if the U.N. gets the approach wrong.

George Russell is executive editor of Fox News