* Potential breakthrough delays vote in Congress on strikes
* Lawmakers seize on Russian proposal as possible way out
* Obama plans national address on Syria (Adds new bipartisan effort, paragraph 4, 20; quotes from hearing 17-19; Pelosi)
By Roberta Rampton and Patricia Zengerle
WASHINGTON, Sept 10 (Reuters) - Top Obama administration officials urged Congress on Tuesday to keep the pressure on Syria over its chemical weapons arsenal while the United States explores a diplomatic alternative to military strikes.
The potential diplomatic breakthrough put the brakes on a planned vote in Congress on the authorization of military force as lawmakers and the administration sought more time to assess Russia's proposal to put Syria's chemical weapons under international control.
President Barack Obama has faced stiff resistance in Congress to any military action, and lawmakers on both sides of the issue were quick to seize on the Russian proposal as a possible way out even as more proclaimed their opposition to U.S. strikes.
A group of Republican and Democratic U.S. senators began drafting a modified resolution on the use of military force that would give the United Nations time to take control of Syria's chemical weapons.
U.S. Secretary of State John Kerry and Defense Secretary Chuck Hagel told Congress the threat of military action was critical to forcing Syrian President Bashar al-Assad to bend on his chemical weapons.
"For this diplomatic option to have a chance of succeeding, the threat of a U.S. military action - the credible, real threat of U.S. military action - must continue," Hagel told the House Armed Services Committee.
The Senate delayed a vote planned for Wednesday authorizing military force. Kerry said Obama might speak to congressional leaders on the "when and how" of an eventual vote.
"Nothing has changed with respect to our request for the Congress to take action," Kerry told the House hearing. "As to when and how, that's something the president may want to chat with the leadership about."
Obama headed to the Capitol to visit Senate Democrats and Republicans in separate meetings on Tuesday, ahead of a nationally televised address he will make from the White House on Tuesday evening.
White House Chief of Staff Denis McDonough told Democratic House members that diplomacy, rather than military action, was the priority now, said U.S. Representative Gene Green, a Texas Democrat.
The Russian diplomatic initiative, which emerged after off-the-cuff remarks by Kerry on Monday alluding to such a deal, marked a sudden reversal following weeks in which the West appeared headed toward intervention in Syria's 2 1/2-year-old civil war.
Senate Republican leader Mitch McConnell became the first of the four top party leaders in the Senate and House to definitively oppose the strikes, saying he would not support a resolution on military force because "a vital national security risk is clearly not in play."
'POTENTIALLY POSITIVE'
House of Representatives Speaker John Boehner, a Republican who announced last week he would support a strike, said the American people still did not support military action in Syria and Obama needed to make a stronger case.
Obama has said Assad needs to be held accountable for an Aug. 21 poison gas attack that killed more than 1,000 civilians, including hundreds of children.
White House spokesman Jay Carney said the president will claim credit for the potential diplomatic breakthrough in his speech. "We see this as potentially a positive development and we see this as a clear result of the pressure that has been put on Syria," Carney said on MSNBC of the Russian proposal.
Kerry told lawmakers that Obama and Russian President Vladimir Putin had discussed the approach last week during the G20 summit in Russia and Obama "directed us to try to continue to talk and see if it is possible."
The House hearing turned contentious at times. Kerry tangled with Republican Jeff Miller of Florida over whether the Senate delayed its vote to explore a diplomatic alternative or because the administration did not have the votes.
Kerry accused Miller of wanting to play politics. "I'm not being political, Mr. Secretary," Miller said. "It's the truth. They don't have the votes. Read any newspaper in this country and you will find that out."
Kerry later shot back: "Look, do you want to play politics here or do you want to get a policy in place?"
The Senate's new bipartisan group working on a modified resolution included Democrats Robert Menendez, Carl Levin, Charles Schumer, Chris Coons and Robert Casey as well as Republicans John McCain, Lindsey Graham, Kelly Ayotte and Saxby Chambliss, aides said.
McCain said the resolution they are preparing would set a strict timeline for Damascus to turn over chemical weapons or face military force. He said he was "extremely skeptical" about such a diplomatic solution but it would be a mistake not to pursue it.
"Some of us are already working on a modification to a congressional resolution that would require strict timelines and strict guidelines that would have to be met as part of the authorization for the president," he said on CBS' "This Morning" program.
Carney also noted "there is ample reason to be skeptical" about Syria's intentions.
"We need to make sure beforehand that the Syrians are serious and will actually follow through on a commitment to give up a chemical weapons stockpile that they've been husbanding for decades against this international prohibition," he said.
The Obama administration will begin discussions with the U.N. Security Council on Russia's proposal on Tuesday. Obama spoke by phone with French President Francois Hollande and British Prime Minister David Cameron about the proposal.
House Democratic leader Nancy Pelosi said the Russian proposal was a validation of Obama's efforts to take action against Syria.
"I think this is a victory for President Obama if it is real," Pelosi told reporters. (Additional reporting by Phil Stewart, Paul Eckert, Susan Heavey, David Lawder and Richard Cowan; Writing by John Whitesides; Editing by Karey Van Hall, Doina Chiacu and Jim Loney)
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