Top US lawmakers began rallying behind President Barack Obama's plan to launch military strikes against Syria to warn the regime and nations like Iran that the world will never condone chemical weapons.
After a passionate plea by US Secretary of State John Kerry not to succumb to "armchair isolationism" after last month's attack in a Damascus suburb, lawmakers drafted a bipartisan measure imposing a 90-day deadline for any US military intervention.
It would also ban the deployment of any US troops on the ground in the war-torn nation, where fighting now in its third year has claimed more than 110,000 lives.
The dramatic developments came as the UN refugee agency released grim new statistics revealing more than two million people had now fled the violence in Syria.
"This is not the time for armchair isolationism. This is not the time to be spectators to a slaughter. Neither our country nor our conscience can afford the cost of silence," Kerry told the Senate Foreign Relations committee.
He warned that other countries such as Iran and North Korea, under fire for its suspect nuclear programs, were closely watching.
"They are listening for our silence," Kerry intoned, during a sometimes heated debate with his former Senate colleagues.
His words were echoed by Defense Secretary Chuck Hagel, who said a US refusal to act after Obama had clearly set chemical weapons use as a "red line" would undermine America's credibility abroad.
"The word of the United States must mean something. It is vital currency in foreign relations and international and allied commitments," Hagel stressed. Both men are due back at the Congress on Wednesday for a further slew of both public and classified briefings.
At earlier White House talks with congressional leaders, Obama said he hoped for "prompt" Congressional votes next week on authorizing "proportional" and "limited strikes" against Syria.
He was speaking before he left late Tuesday for Sweden and a G20 summit in Russia, where he will lay out the US case for strikes.
After the nearly four-hour hearing,the Senate committee re-worded the resolution put forward by the White House to restrict it to "limited and tailored use of the United States Armed Forces against Syria, according to a copy of the draft obtained by AFP.
The authorization provided by the resolution "shall terminate 60 days after the date of the enactment of this joint resolution, except that the president may extend, for a single period of 30 days, such authorization" if he deems it necessary.
It did "not authorize the use of the United States Armed Forces on the ground in Syria for the purpose of combat operations."
House Speaker John Boehner and House Majority Leader Eric Cantor -- leading Republicans who have had frosty relations with Obama on domestic policy -- have now both said they would support his plan.
"This is something that the United States as a country needs to do," Boehner said, calling on Republican colleagues to follow his example.
But in a sign of the deep public misgivings over wading into another foreign conflict, the hearing was interrupted several times by protesters.
Two polls released Tuesday showed strong opposition to a US military intervention in the crisis. Some 48 percent of Americans told a Pew Research Center survey that they opposed "conducting military air strikes" with only 29 percent in favor.
A poll by the Washington Post-ABC found a similar margin of nearly six in 10 Americans opposed to missile strikes.
The Syrian opposition meanwhile said it feared a fresh chemical attack by the regime of Syrian President Bashar al-Assad, after spotting three convoys of vehicles believed to be filled with such arms.
The Syrian army had also retaken control of the strategic town of Ariha in northwest Syria after 10 days of intense bombing and clashes, the Syrian Observatory for Human Rights sid.
An influential Republican Senator John McCain lambasted the administration for delaying its response for so long, before now signalling its intent.
"You tell the enemy you're going to attack them, they're obviously going to disperse and try to make it harder," he said.
Obama said the August 21 attack, which Washington says involved the use of sarin gas, posed a serious national security threat to the United States and its allies.
"As a consequence, Assad and Syria needs to be held accountable," he said, while assuring Americans he would not use ground troops.
Kerry stressed the aim of any strikes would be to degrade Assad's military capabilities.
But he seemed to indicate that the administration would like to preserve the option of sending in troops "in the event Syria imploded, for instance, or in the event there was a threat of a chemical weapons cache falling into the hands" of Al-Qaeda-linked fighters.
UN leader Ban Ki-moon meanwhile warned that a western military strike could make things worse.
"We must consider the impact of any punitive measure on efforts to prevent further bloodshed," Ban said.