Updated

Insurgents in the hostile city of Fallujah have continued to ignore orders to turn in their weapons, prompting U.S. military officials to warn an offensive may be near.

Guerrillas and residents handed over only a paltry assortment of old and rusty weapons by Thursday, and coalition officials said the city's shaky cease-fire may soon collapse.

"We weren't pleased at all with the turn-in [of weapons] we saw yesterday. In terms of volume, it amounted to about a pick-up [truck] full," Lt. Gen. Jim Conway (search) said, characterizing the turned-in weapons as "junk."

"It's our estimate the people of Fallujah have not responded well to the agreements ... that the weapons turn-in would be a reflection of their desire to end the situation peacefully," Conway added. "I think what happens next is in the hands of the negotiators."

U.S. officials said the weapons deal's success hinged on whether the Fallujah negotiators —a group of local civic leaders — could convince the guerrillas to comply.

Enemy forces in Fallujah had "days, not weeks" to turn in heavy weapons, Brig. Gen. Mark Kimmitt (search) told Fox News, warning that patience was running thin and that U.S. forces would go back on the offensive if more weapons weren't handed over.

Non-cooperation could mean a resumption of fighting in Fallujah, a Sunni stronghold that has proven to be a bastion of resistance against coalition forces. U.S. military officials say Marines are only firing defensively if fired upon and even then, it's not in every instance.

"We have been underwhelmed by the seriousness of the weapons turnout effort," Dan Senor, spokesman for the top U.S. civilian administrator in Iraq, L. Paul Bremer, told Fox News on Thursday. "We've been told to wait a few days here" before going back on the offensive.

"It remains to be seen whether or not they [rebels and city leaders] can actually deliver," Senor added.

A U.S. military general in Baghdad sent Fox News photos of old and rusty weapons that rebels handed over to the U.S. military. Marines said few weapons had been turned in and that most that had been were old or didn't work.

The handover was supposed to be part of an agreement in which city leaders were to persuade insurgents to hand over heavy weapons in return for a U.S. promise to not storm the city and instead allow the return of families that had fled.

"These may be early indications that the insurgents may not be living up to the requirements of the agreement," said Marine Lt. Col. Brennan Byrne.

The weapons collected are a "signal that the turnover is not proceeding in good faith," Togo West, former Army secretary, told Fox News.

"Our forces are going to have to decide whether they can leave that stronghold in the hands of those causing the struggle," he said. "I think they'll [Marines] probably wait a little longer to see if there can be any further successes. If not, they're just gonna have to go ahead and do their job."

On Thursday morning, Marines stopped letting residents return to Fallujah. About 10 families had made it back into the city before another 600 Iraqis waiting at a Marine checkpoint were told that no more would be allowed to enter.

The terms of the deal called for all large weapons to be handed in: machine guns, mortars, rocket-propelled grenades, improvised explosive device and the like. But the weapons being turned in did not meet those descriptions, with some even being duds, suitable for training purposes only.

Conway said U.S. forces wanted the "good people" of Fallujah to turn in rebels in the city to avoid more violence.

"[If] we can do that, a lot of innocent people can avoid being hurt. If the negotiators can cause that separation, then we will deal with what's left," Conway said.

He said he agreed with the political effort to find a peaceful solution, but said that it had halted the military momentum.

"We had momentum on our side and our losses were very small until the point we were told to halt and allow negotiations to take place," Conway continued. "I think those [losses] will be more significant if we do continue. But that's just the military losses and doesn't take into account civilians and those caught between."

The New York Times reported that U.S. military commanders have pulled in reinforcements from the western desert to build a force of more than 3,500 around Fallujah.

Marines are conducting raids in the suburbs to kill or capture fighters, find weapons and dry up support for militants, one officer said. The units are restocking several days' worth of food, fuel, water and ammunition.

Super Cobra attack helicopters and Air Force AC-130 gunships prowling the night skies will soon be joined by several AV-8B Harrier attack jets, the Times reported. In the past month, the Air Force has doubled to 50 the number of strike missions flying on call over Fallujah and other western cities.

U.S. forces are primarily hunting for those responsible for the grisly treatment of the corpses of four American contractors (search) on March 31, when their charred bodies were dragged through the streets of Fallujah and hung from a bridge.

U.S. forces then promised an "overwhelming response" to that incident.

Conway said the cease-fire was violated Wednesday by a group of between 60 or 80 insurgents on the northwest side of Fallujah.

He said he believes there are a "hardcore of a couple of hundred" foreign fighters and several hundred others "influenced by their imams and the idea of jihad."

Conway reiterated coalition comments that the insurgents had been using mosque minarets as sniper positions, that the insurgents were using ambulances to ferry fighters and weapons inside Fallujah and that mosques were being used as command-and-control centers.

Mosques are traditionally protected in times of war and not to be fired upon. But insurgents have been using the buildings as protection while firing on coalition forces, thereby stripping the mosques of their protective status.

Conway said a tight cordon was in place around Fallujah, which he hoped would prevent fighters from slipping away.

When asked if he thought insurgents would surrender, Conway replied: "They want to fight as long as they can and go down for the cause, and that's what I think we're dealing with. But, of course, if they want to turn themselves in, then we would be delighted."

Fighting in Fallujah has killed at least seven U.S. Marines and more than 600 Iraqis, mostly civilians, according to the city hospital.

Fox News' Owen Fay contributed to this report.