Senior U.S. military officials have tried to reassure lawmakers on Capitol Hill about the Pentagon's ability to sustain adequate troop levels in Iraq and Afghanistan.

In the wake of a recent Pentagon decision to call up several thousand members of a little used group of reservists, members of Congress are asking new questions about force structure and troop rotations.

In a hearing Wednesday of the House Armed Services Committee, lawmakers expressed concern about strains on the army and marines.

Congressman Ike Skelton, the ranking Democrat on the committee, said troops continue to respond to difficult tasks in Iraq and Afghanistan, but said this to Pentagon and military officials.

"I've got to tell you I'm worried, worried for them," he said. "We are asking very few to exert an enormous sustained effort for the good of all of us here in our country and we need to pay very strict attention to that."

The Pentagon decision to use members of the Individual Ready Reserve involves about 5,600 soldiers with specific skills who are still subject to emergency service.

The last time such a large number of troops were called up from this reserve group was during the Gulf War in 1991.

Opening the hearing, Congressman Duncan Hunter, the Republican Committee chairman who recently returned from Iraq, underscored concern among Republicans and Democrats about Pentagon reliance on reservists.

"[We are concerned about] the continued heavy reliance on the reserve components, which we understand will make up 43 percent of the force going into Iraq, and the reasons behind the army's need to involuntarily mobilize 5,600 people in the Individual Ready Reserve," he added.

In their testimony, Pentagon officials and military commanders acknowledged increased pressure on troops, but said force levels in both places are still seen as sufficient by top commanders in Iraq and Afghanistan.

David Chu, Undersecretary of Defense for Personnel and Readiness, defended use of the Ready Reserve. "Individual Ready Reserve service is, of course, part of the obligation of military service that each entrant in the military assumes. The fact that it is rare that we call up an Individual Ready Reservist does not mean it is inappropriate. It is important to our military readiness," he noted.

Army Vice Chief of Staff, General Richard Cody, had this response when asked about media reports about strains on members of the military reserve and the military in general.

"We are building back our surge capability right now," he said. "Are we stretched thin with our active and reserve component forces right now? Absolutely. We just did the largest move of the army since World War II. You can't move eight-and-a-half divisions and 240,000 soldiers without stressing the force."

The U.S. military is now engaged in its third rotation of troops in Iraq, and its sixth in Afghanistan, where there are respectively approximately 140,000 and 17,000 troops.