* Voices skepticism in White House speech
* Potential breakthrough delays vote in Congress on strikes
* Lawmakers seize on Russian proposal as possible way out (Updates with comments from Obama speech)
By Steve Holland and Jeff Mason
WASHINGTON, Sept 10 (Reuters) - U.S. President Barack Obama vowed to pursue a diplomatic initiative from Russia over Syria's chemical weapons on Tuesday but voiced skepticism about it and urged Americans to support his threat to use military force.
Obama said in a White House speech that a Russian offer to push Syrian President Bashar al-Assad to place chemical weapons under international control offered the possibility of heading off the type of limited military strike he is considering against Syria.
Speaking from the White House's East Room, Obama said U.S. and Russian officials would keep talking about the initiative and that he would discuss it with Russian President Vladimir Putin.
Meanwhile, he said, he has asked the U.S. Senate to put off a vote on his request for an authorization of military force to let the diplomacy play out. He set no timetables for action, but said any deal with Assad would require verification that he keep his word.
"It's too early to tell whether this offer will succeed. And any agreement must verify that the Assad regime keeps its commitments. But this initiative has the potential to remove the threat of chemical weapons without the use of force, particularly because Russia is one of Assad's strongest allies."
The Russian offer put the brakes on a vote in Congress over authorizing military force as lawmakers and the administration sought more time to assess Russia's proposal to put Syria's chemical weapons under international control.
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, despite skepticism about its eventual success.
Obama used much of his speech to lay out the case against Syria, saying there was plenty of evidence showing that the Syrian government was behind an Aug. 21 chemical weapons attack that killed 1,429 people, including more than 400 children.
He argued that Syria should face consequences for using such weapons because much of the world has long since adopted a ban on chemical weapons and that if the civilized world does nothing to respond, it will only embolden U.S. adversaries.
"If we fail to act, the Assad regime will see no reason to stop using chemical weapons," said Obama.
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. (Additional reporting by Mark Felsenthal.; Editing by Alistair Bell and Christopher Wilson)