WASHINGTON – The National Guard and Reserves will play a much smaller role next year in Iraq and Afghanistan, dropping to less than one fifth of overall American forces there, the nation’s top military officer told senators yesterday.

In response to repeated questions about the strain on reserve forces on the front lines, General Peter Pace, chairman of the Joint Chiefs of Staff, told the Senate Armed Services Committee that Guard and Reserves will make up just 19% of the forces deployed in Iraq and Afghanistan over the next year.

Currently they make up about 30% of American forces in those countries, General Pace said. That means the planned reduction would lower the reserves’ proportion of total American forces in Iraq and Afghanistan by about one-third.

There are now about 138,000 American troops in Iraq and 19,000 more in Afghanistan.

The planned reduction in reserves comes as Bush administration officials have been saying they hope to reduce the numbers of American troops in Iraq this year, assuming the Iraqi government and its forces can take a wider role in the war and keeping order.

The Bush administration has been under pressure to bring more American troops home. A study commissioned by the Pentagon said last month that the wear and tear of American deployment in Iraq was beginning to wear down the Army and questioned how much longer it could continue operating there at full effectiveness.

Reserve forces have made up as much as 40% of American forces in Iraq and Afghanistan. The number of Army National Guard soldiers in Iraq and Afghanistan peaked at 69,416 last September, with most in Iraq.

General Peter Schoomaker, the top Army officer, told the senators that heavy use of the Guard and Reserve has given the Army time to reorganize and prepare for its regular troops to take on a broader role beginning in March.

Defense Secretary Rumsfeld, meanwhile, told the panel that continued corruption in Iraq could damage efforts to create a democracy there and it is up to the Iraqis to seize control and take more responsibility for their country.

“It’s true that violence, corruption, and criminality continue to pose challenges in Iraq” and are “so corrosive of democracy,” he said.

“It’s critically important that it be attacked and that the new leadership in that country be measured against their commitment to attack corruption,” he added.

Mr. Rumsfeld provided no specific examples. But there have been recent allegations that some revenue from Iraq’s slowly rebuilding oil industry have been siphoned to help finance the insurgency there.

Mr. Rumsfeld added that “our awareness of corruption is increasing,” because coalition officials are doing more to investigate those problems within the government.

Committee chairman John Warner, a Republican of Virginia, said the next six months would be critical in Iraq and will be key to the eventual withdrawal American troops and coalition forces. He said increasing problems with corruption in Iraq make it more difficult for coalition forces.

Mr. Rumsfeld faced criticism from one committee member who said the $439.3 billion defense budget for 2007 that President Bush proposed on Monday is not responsible because it doesn’t reflect the billions of dollars that will be spent on Iraq and Afghanistan.

Senator Levin, a Democrat of Michigan, also questioned whether America will be able to sustain necessary troop levels in Iraq, where difficult conditions are wearing down both people and equipment. A report conducted for the Pentagon drew a similar conclusion last month.

Mr. Bush proposed an initial payment of $50 billion for those wars next year, compared with $120 billion the administration says will be needed this year.

Mr. Rumsfeld also said the military must continue to change in order to defend the nation against enemy terrorists who could acquire a nuclear weapon or launch a chemical attack against a major American city, Mr. Rumsfeld told the Senate Armed Services Committee.

“No nation, no matter how powerful, has the resources or capability to defend everywhere, at every moment of the day or night, against every conceivable type of attack,” Mr. Rumsfeld said.

He said Mr. Bush’s new Pentagon budget – a 7% boost over this year – would help pay for the military’s transformation into a more flexible fighting force. Transformation includes large spending increases for special operations forces, unmanned vehicles, foreign language skills, and gathering intelligence.