WASHINGTON – Dealing with a civil war in Iraq would be the responsibility of Iraq’s own security forces, at least initially, Defense Secretary Rumsfeld told Congress yesterday.

Testifying alongside senior military leaders and Secretary of State Rice, Mr. Rumsfeld said he did not believe Iraq would descend into all-out civil war, though he acknowledged that sectarian strife had worsened.

General John Abizaid, the top American commander in the Middle East, said the situation in Iraq had evolved to the point where Sunni-Shiite violence was more of a threat to American success there than the insurgency, which continues taking a deadly toll on Iraqi and American troops, and to impede efforts to stabilize the country.

Mr. Rumsfeld previously had been reluctant to say what the American military would do in the event of civil war, but in an appearance before the Senate Appropriations Committee he was pressed on the matter by Senator Byrd, a Democrat of West Virginia.

“The plan is to prevent a civil war, and to the extent one were to occur, to have the – from a security standpoint – have the Iraqi security forces deal with it, to the extent they are able to,” Mr. Rumsfeld told the committee.

He did not elaborate on the implication of his remark: that at some point the Iraqi security forces might be overwhelmed by a civil conflict and ask the Americans to get involved militarily.

One of Mr. Rumsfeld’s chief critics in Congress, Senator Kennedy, a Democrat of Massachusetts, issued a statement after the hearing urging the administration to explain more fully what it would do in case of a civil war.

“Obviously, it’s not realistic to depend on the Iraqi security forces, which are not yet able to fight on their own,” Mr. Kennedy said. “So, Secretary Rumsfeld is basically saying that if the prevention strategy fails and Iraq plunges into civil war, U.S. troops will inevitably be deeply involved.”

Mr. Rumsfeld said the key to avoiding civil war is for Iraq’s political leaders to form a government of national unity, he said.

Both General Abizaid and Mr. Rumsfeld cited progress in the training of Iraqi security forces. General Abizaid said more than 100 Iraqi battalions are now conducting counterinsurgency operations, compared with only five in 2004. He did not mention that the number of Iraqi battalions rated as capable of operating without American military assistance had recently dropped from one to zero.

During an extensive question-and-answer session with committee members, some Democrats including Mr. Byrd and Senator Kohl of Wisconsin sharply criticized the war but the overall tone of the hearing was not hostile.