By Steve Holland

WASHINGTON (Reuters) - President Barack Obama vowed on Saturday that Syria will be held to account if it fails to live up to its promises to surrender chemical weapons as he faced questions about how a deal brokered by U.S. and Russian diplomats would be enforced.

In a statement, Obama said a framework deal was an important, concrete step toward getting Syria's chemical weapons under international control so they can ultimately be destroyed. The deal emerged from Geneva talks between U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov.

"While we have made important progress, much more work remains to be done," said Obama.

Obama has been bombarded with criticism for his handling of Syria and a muddled message. First, he took U.S. forces to the brink of a military strike over an August 21 poison gas attack in Syria that Washington blames on Syrian President Bashar al-Assad. He then asked Congress to authorize the strike, but less than a week later requested lawmakers hold off on a vote to allow diplomacy more time.

He now faces questions about how the Syrian diplomatic deal will be enforced. Senior administration officials said on Friday the United States will not insist that the use of military force be included among the consequences Syria would face in a U.N. Security Council resolution being negotiated, in order to avoid a Russian veto.

Obama, in his statement, insisted that the United States "remains prepared to act" should diplomatic efforts fail.

But Republican Senators John McCain and Lindsey Graham, who have sharply criticized Obama's handling of Syria, said the deal is meaningless unless backed up with the threat of military force.

"It requires a willful suspension of disbelief to see this agreement as anything other than the start of a diplomatic blind alley," they said.

But Nancy Pelosi, leader of the Democratic minority in the House of Representatives, disagreed. She said the agreement will allow for enforcement under the U.N. charter's Chapter 7, which covers the use of military force.

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"The firm and united response agreed upon today to end Syria's deadly use of chemical weapons was only made possible by a clear and credible threat of the use of force by the United States," said Pelosi.

Obama said the United States will continue working with Russia, the United Kingdom, France, the United Nations and others to "ensure that this process is verifiable, and that there are consequences should the Assad regime not comply with the framework agreed today."

"In part because of the credible threat of U.S. military force, we now have the opportunity to achieve our objectives through diplomacy," he added.

U.S. forces were still positioned for possible military strikes on Syria.

"We haven't made any changes to our force posture to this point," Pentagon spokesman George Little said in a statement Saturday.

Obama, briefed on the results of the Geneva talks by his national security adviser, Susan Rice, said he had spoken to both Kerry and the U.S. ambassador to the United Nations Samantha Power, who will lead U.S. efforts on the U.N. negotiations.

(Additional reporting by Phil Stewart; Editing by Doina Chiacu)