ALSO READ: Russia tells US it needs UN approval for Syria strike Appearing before the House Foreign Affairs Committee, Mr. Kerry offered a new argument: extremist groups fighting against the Syrian government would become stronger if the United States did not carry out a military strike.
Mr. Kerry said the United States had worked hard in recent months to persuade Arab nations and benefactors not to finance or arm the more extremist rebels who are battling Mr. Assad's forces. But if the United States does not punish the Assad government, Mr. Kerry said, it is likely that some Arab supporters of the Syrian opposition will provide arms and financing to the best rebel fighters, regardless of whether they are extremists. "We will have created more extremism and a greater problem down the road," Mr. Kerry said.
After days of discussion over whether a limited military strike would be effective, administration officials sought to assure anxious lawmakers that it would not provoke a major escalation in the fighting.
Representative Christopher H. Smith, a New Jersey Republican, asked if a missile attack might set off a chain reaction that could lead to a military action as prolonged as the 78 days of NATO bombing in Kosovo. "How do you define limited and short duration?" he asked. "And what might Assad do in retaliation?"
General Dempsey acknowledged that was a risk but argued that the danger had been mitigated since the United States had signaled that it was planning a limited strike, even as it retained the ability to carry out additional attacks if Mr. Assad responded in a provocative manner. "We're postured for the possibility of retaliation," he said.
The most heated moment in the hearing came when Representative Jeff Duncan, a South Carolina Republican, accused Mr. Kerry of taking a hawkish stand on Syria while ignoring the terrorist attack on the American Mission in Benghazi, Libya.
"Mr. Kerry, you have never been one that has advocated for anything other than caution when involving U.S. force in past conflicts," Mr. Duncan said. "Is the power of the executive branch so intoxicating that you would abandon past caution in favor for pulling the trigger on a military response so quickly?"
His voice rising with anger, Mr. Kerry responded that as a senator, he had supported "military action in any number of occasions," citing the invasions of Panama and Grenada. Mr. Kerry also voted in favor of President George W. Bush's invasion of Iraq in 2003, before turning against the war.
"We're talking about people being killed by gas, and you want to go talk about Benghazi," Mr. Kerry said.
In an indication of the hostility that Russia has shown to any American military action, President Vladimir V. Putin accused Mr. Kerry of lying to Congress.
"They lie beautifully, of course," Mr. Putin said in remarks that were televised in Russia. "I saw the debates in Congress. A congressman asks Mr. Kerry, 'Is Al Qaeda there?' He says, 'No, I am telling you responsibly that it is not.' "
"Al Qaeda units are the main military echelon, and they know this," Mr. Putin said. "It was very unpleasant and surprising for me - we talk to them, we proceed from the assumption that they are decent people. But he is lying and knows he is lying. It's sad."
Peter Baker contributed reporting from Stockholm, and David M. Herszenhorn from Moscow.
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