The Bush administration is conceding for the first time that the United States may not finish a complex security agreement with Iraq before President Bush leaves office.

Faced with stiff Iraqi opposition, it is "very possible" the U.S. may have to extend an existing U.N. mandate, said a senior administration official close to the talks. That would mean major decisions about how U.S. forces operate in Iraq could be left to the next president, including how much authority the U.S. must give Iraqis over military operations and how quickly the handover takes place.

The official said the goal is still to have an agreement by year's end. And the U.S. ambassador to Iraq, Ryan Crocker, said he feels no pressure from the U.S. political calendar, and that Dec. 31 is "a clear deadline."

Still, Crocker also said last week, "My focus on this is more on getting it done right than getting it done quick."

The Bush administration is seeking an agreement with Baghdad that would provide for a normal, permanent U.S. military and diplomatic presence in Iraq. The word "permanent" has been a flashpoint for many who oppose the war, both in the U.S. and Iraq. But the U.S. official stressed that the agreement will not call for permanent U.S. bases on Iraqi soil.

Instead, the proposed agreement would allow U.S. troops or personnel to operate out of U.S., Iraqi or joint facilities through either short or long-term contracts, said the official, who spoke on condition of anonymity because the negotiations are not public.

"The idea that the U.S. will have a normal, diplomatic and military presence, and need access to facilities — not necessarily our facilities, but need facilities — is permanent," said the official, who is close to the ongoing talks.

Those facilities, the official said, could belong to the Iraqis, and the U.S. would simply be using them on a renewable basis. Or they could be existing U.S. facilities that over time would be taken over by the Iraqis.

U.S. and Iraqi leaders are struggling to negotiate two parallel agreements. One would lay out broad, long-term political, economic and security ties between the U.S. and Iraq, setting up a normal state-to-state relationship.

The second — and decidedly more difficult pact — is the Status of Forces Agreement that would detail the legal basis for the ongoing presence of U.S. military forces operating in Iraq. The agreements would replace a United Nations mandate that has been in place since the U.S.-led invasion in March 2003, but expires at the end of this year.

The U.S. and Iraq may be able to map out the broader document describing the two countries' long-term relationship, but may have to extend the current U.N. mandate because many of the thornier military details will require more time, the U.S. official said.

On Monday, Sens. Carl Levin and John Warner sent a letter to Secretary of State Condoleezza Rice urging the administration to "be more transparent, with greater consultation, about the progress of these deliberations."

"The situation in Iraq is extremely complex and Congress ... has legitimate concerns" about the agreements, wrote Levin, D-Mich., and Warner, R-Va. Levin is chairman of the Senate Armed Services Committee; Warner is a member.

Iraqi officials have raised a number of objections to the draft documents, both publicly and privately. And they are now suggesting that the latest proposal isn't even worth submitting to their parliament for approval.

On Monday two Iraqi lawmakers who saw the proposed draft said the document, put forward Sunday, said it seeks to address some of Iraq's concerns. It adds an explicit promise that U.S. forces in Iraq will not attack neighboring countries and that Iraqi authorities will be notified in advance of any action by U.S. ground forces, the lawmakers said.

While it gives U.S. forces the power to arrest suspects, it says any detainees would be handed over to Iraqi authorities, said the lawmakers, Mahmoud Othman and Iman al-Asadi.

Hadi al-Amri, head of the Badr Organization, a pro-government Shiite party with close ties to Iran, said the latest draft was still unacceptable, and warned that the positions and interests of the two sides are so far apart that any kind of agreement is "impossible."

Iraqi Prime Minister Nouri al-Maliki has been meeting with Iranian leaders in Tehran this week, and Iran's supreme leader voiced strong opposition to the U.S.-Iraqi security pact.

The Iranians fear the deal would solidify U.S. influence in Iraq and give American forces a launching pad for military action against them. Al-Maliki tried to ease Tehran's concerns, assuring the Iranians that a deal would pose no threat to their security.

Preserving Iraq's sovereignty is the critical underpinning of the agreement, affecting all of the more specific, and troublesome issues. For example, while the Iraqis want control of their own air space, the U.S. questions whether Iraq commanders should have to approve every aircraft flight or combat operation.

Iraq has been expanding and improving its military, but approving each of the many U.S. troop movements that go on daily all across the country likely would be beyond its current capability.

Another option would be for U.S. commanders to coordinate movements with the Iraqis, but only require approval for certain operations. A key priority for Washington is to maintain the U.S. military's ability to gather intelligence and conduct counterterrorism activities.

One way to resolve the matter would be to include provisions in the agreement that would expire, or require renewal every year or two. That would allow a gradual transition to greater Iraqi control as the country advances.

One key sticking point involves the Iraqis' desire to end the blanket immunity that U.S. contractors now enjoy. But U.S. officials believe that security contractors working for the State Department and Defense Department should continue to be immune under Iraqi law.

The issue gained notoriety last year after a shooting incident involving Blackwater Worldwide guards left 17 Iraqi civilians dead.

A second point involves coalition forces currently serving in Iraq. The security agreement would only apply to U.S. forces, so the Iraqis would have to decide how to deal with troops from other countries that are included in the U.N. mandate.