The Pentagon is considering a plan to extend the tours of duty for up to 15,000 U.S. troops serving in Iraq, a defense official said Monday.

The idea is among options being considered in response to a request in the last couple of weeks by Gen. David Petraeus, U.S. commander in Iraq, said the official who spoke on condition of anonymity because the idea has not been approved.

Because Petraeus believes the troop increase President Bush announced in January has produced some momentum in fighting violence in Iraq, Petraeus wants to maintain troops at that level past the summer, the official said.

Defense officials are looking at the idea of a maximum 120-day extension for five active duty brigades that would otherwise come home in the coming months — four ground units and one aviation combat brigade totaling roughly 15,000 troops, the official said. The plan would have to be approved by Defense Secretary Robert Gates.

Other options also are on the table. The official declined to name them but others have said previously that sending some troops earlier than planned also would be considered.

There are currently some 145,000 U.S. troops in Iraq.

Also on Monday, officials said some 13,000 National Guard troops are receiving notice to prepare for possible deployment to Iraq, which would be the second tour for several thousand of them.

The orders had been anticipated, but the specific units were not announced until Monday. They are the Army National Guard's 39th Infantry Brigade Combat Team, based in Little Rock, Ark.; 45th Infantry Brigade Combat Team, Oklahoma City; the 76th Infantry Brigade Combat Team, Indianapolis, and the 37th Infantry Brigade Combat Team, Columbus, Ohio.

The Guard units would serve as replacement forces in the regular troop rotation for the war, and would not be connected to the recent military buildup for security operations in Baghdad, the Pentagon said.

One unit would deploy in December and the others in 2008, the Army said.

"They are receiving alert orders now in order to provide them the maximum time to complete their preparations," the Defense Department said in a separate statement. "It also provides a greater measure of predictability for family members and flexibility for employers to plan for military service of their employees."

The final determination on whether the Guard units will deploy will be made based on conditions in Iraq, officials said.

The troop alerts and word of possible extensions come as Bush and Congress wrestle over legislation that would set timelines for troop withdrawals from Iraq.

Bush asked for more than $100 billion to pay for the wars in Iraq and Afghanistan this year. Congress has approved the money, but the Senate added a provision calling for most U.S. combat troops to be out of Iraq by March 31, 2008. The House version demands a September 2008 withdrawal. Bush has said he would veto any legislation that includes such deadlines.

Republicans, who say they can uphold Bush's veto, are trying to pressure Democrats into stripping out the contentious Iraq language. Republican leaders in both the House and Senate on Monday called on House Speaker Nancy Pelosi to cut short the House's two-week recess and return immediately to finish work on the bill.

"It should go without saying that our military leaders are in the best position to know the needs of our troops, and they have left no doubt that this funding is needed urgently," the GOP leaders wrote.

Democrats say they are not budging and that Bush will have a bill before the end of the month. Pelosi, D-Calif., said in an interview Friday that she had no intention of cutting the recess short.

Senate Majority Leader Harry Reid, D-Nev., said that four years after the fall of Baghdad Iraq U.S. troops "desperately need a postwar strategy that recognizes the political situation on the ground and removes them from policing a civil war."