WASHINGTON — President Bush's planned escalation of U.S. forces in Iraq will require as many as 28,500 troops, Pentagon officials told a Senate committee Thursday.

Deputy Defense Secretary Gordon England also told the Senate Budget Committee that it will be clear within months whether the so-called surge in forces has succeeded in helping secure Iraq.

"By this summer we would have a much better indication in terms of the success of the program," England said. "And so at that time we would adjust however is appropriate to do so."

In January, Bush said he would send 21,500 more combat troops to Iraq. England said 6,000 to 7,000 support troops will be needed to back up the larger combat force.

England's estimate differed from the Congressional Budget Office's estimate last month that as many as 28,000 extra troops would be needed to support the escalation.

Requests already have been granted for 2,400 support troops, said Adm. Edmund Giambastiani, vice chairman of the Joint Chiefs of Staff. He said there are additional requests for 4,000 more.

Existing logistics bases, many of them in and around Baghdad, will limit the number of new support troops needed, Giambastiani said.

There are about 10,000 soldiers in Iraq now associated with the escalation, according to Lt. Col. Carl Ey, an Army spokesman. In all, there are about 140,000 U.S. troops now in Iraq.

The increase in troops is expected to peak in May, according to the Congressional Budget Office.

Pentagon officials stressed that the additional troops will be properly equipped. Army Brig. Gen. Charles Anderson, director of force modernization, said in a briefing this week that they will have the body and vehicle armor they need.

Giambastiani also said the wars in Iraq and Afghanistan had affected the military's ability to deal with threats from other countries but not its ability to win a conflict.

"What will happen is there will be some added risk with regard to the time it takes us to be victorious or to win," he said. "And, potentially, you'll have some additional casualties. But let there be no mistake: We can get our job done if another crisis comes about."

Meanwhile, top members of the Senate Budget Committee scoffed at Pentagon estimates that the escalation would cost $5.6 billion. By contrast, the CBO estimated the cost of sustaining 35,000 troops there, and determined it could be as much as $20 billion.

"Put me down as a skeptic on the $5.6 billion," said Senate Budget Chairman Kent Conrad, D-N.D.

"It's obvious the $5.6 billion is a number that's not accurate," added Sen. Judd Gregg, R-N.H., the panel's top Republican.

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