After a passionate plea by US Secretary of State John Kerry not to succumb to "armchair isolationism" after last month's suspected chemical weapons 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.
After British lawmakers voted down a bid to take any military action against Syrian President Bashar al-Assad's regime, Washington is looking elsewhere to build an international coalition of the willing.
France will hold an emergency parliamentary debate on the Syria crisis on Wednesday but Prime Minister Jean-Marc Ayrault has ruled out a vote.
French President Francois Hollande can order military action without parliamentary approval though some lawmakers have urged him to put the issue to a vote.
Nearly three quarters of French people want any potential military intervention in Syria to be put to a vote in parliament, according to a poll Tuesday.
Paris has urged its European Union partners to unite in response to the Syria crisis, as Paris pushes for punitive military strikes against the regime.
"Europe must also unite on this issue. It will do so, each with its own responsibility. France will assume its own," Hollande said during a joint press conference with German counterpart Joachim Gauck.
"When a chemical massacre takes place, when the world is informed of it, when the evidence is delivered, when the guilty parties are known, then there must be an answer," Hollande added.
Obama has deferred any military action in Syria, seeking Congressional approval at a vote scheduled for September 9.
The US Senate committee, after a nearly four-hour hearing, 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.
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.
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 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.
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.