US President Barack Obama won the backing of key Republican leaders Tuesday for military strikes on Syria, as his top aides urged skeptical US lawmakers to punish Damascus over chemical weapons attacks.

Secretary of State John Kerry, testifying before a Senate committee, warned against "armchair isolationism" after a chemical weapons attack last month in a Damascus suburb, which the US says killed more than 1,400 people.

The dramatic developments in Washington came as the UN refugee agency released grim new statistics revealing more than two million people had now fled the violence in the war-torn country.

Obama said he hoped for "prompt" votes next week on authorizing force against Syria as he met congressional leaders at the White House, and said he was sure he would secure the necessary support.

The Republican speaker of the House, John Boehner -- who has fought tooth and nail with Obama over domestic policy -- emerged an hour later offering a firm endorsement of his rival's strategy.

"I am going to support the president's call for action," Boehner said.

"This is something that the United States as a country needs to do," he added, calling on Republican colleagues to follow his example.

Another key Republican, House Majority Leader Eric Cantor -- popular with the party's conservative rank and file -- also backed Obama.

"Assad's Syria, a state sponsor of terrorism, is the epitome of a rogue state, and it has long posed a direct threat to American interests and to our partners," Cantor said.

As the White House stepped up its blitz to win over skeptical lawmakers, Kerry told a key Senate panel that "we must stand up and act" after the chemical weapons attack.

"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," he said before the Senate Foreign Relations committee.

But in a sign of deep public misgivings, he was met with protests as he entered the packed room when a man with a pink shirt yelled "say no to war in Syria" adding: "We cannot afford to have another war, we need health care."

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.

But the Democratic chairman of the Senate committee, Robert Menendez, said "there are risks to action but the consequences of inaction are greater and graver still."

The Republican-controlled House, which will hear from top administration officials on Wednesday, is however seen as the tougher sell for Obama who has asked Congress to approve the strikes.

Democratic House leader Nancy Pelosi said she wanted more information about the US intelligence on the August 21 attack, but she appeared to be leaning towards a yes vote.

"President Obama did not draw the red line. Humanity drew it decades ago," Pelosi said.

Obama said the attack, which Washington says involved the use of sarin, 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.

Defense Secretary Chuck Hagel insisted before the Senate panel that "we have made clear that we are not seeking to resolve the underlying conflict in Syria through direct military force."

But Assad warned in an interview with Western media released Monday that strikes of any kind could set off a wider Middle East conflict.

"Everyone will lose control of the situation once the powder keg explodes. Chaos and extremism will spread. There is a risk of regional war," Assad told French newspaper Le Figaro.

UN leader Ban Ki-moon also 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.

More than 100,000 people have died since the rebellion to oust the long-time leader erupted in March 2011.

The UN refugee agency Tuesday revealed that some two million Syrians have now fled, in a tide of humanity which is straining resources in neighboring countries. Millions more have been displaced inside Syria.

Antonio Guterres, the United Nations High Commissioner for Refugees, described the figures as a "disgraceful humanitarian calamity with suffering and displacement unparalleled in recent history."

Amid the mounting tension, Israel and the United States launched a missile over the Mediterranean as part of a joint exercise.

However, the Pentagon said the test was not linked to any possible US military action against Syria.

France, which backs Obama in his determination to launch a military intervention, on Tuesday called on Europe to unite in its response to the crisis.

"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," French President Francois Hollande said.

"This answer is expected from the international community," he said.

France has emerged as the main US ally in the Syria crisis after the British parliament last week rejected involvement in any military action.