John Kerry: "Assad regime unleashed an outrageous chemical attack"
'Chemical attacks': What we know
President Barack Obama has won backing from key US political figures on his plans for a military strike on Syria.
Mr Obama said a "limited" strike was needed to degrade President Bashar al-Assad's capabilities in response to an alleged chemical weapons attack.
Key Republican leaders John Boehner and Eric Cantor both voiced their support for military action. Congress is expected to vote next week.
The UN earlier confirmed that more than two million Syrians were now refugees.
More than 100,000 people are thought to have died since the uprising against President Assad began in March 2011.
However much Republicans dislike the president, they do not want to leave the US in a position where so many have already argued it would be weakened in the eyes of the world”
Mark MardellNorth America editor
Read more from Mark
President Obama and Vice-President Joe Biden met House Speaker John Boehner, House Democratic Leader Nancy Pelosi and the chairmen and ranking members from the national security committees in Washington on Tuesday.
Mr Boehner said his supported Mr Obama's call for action, and that only the US had the capacity to stop President Assad. Mr Boehner urged his colleagues in Congress to follow suit.
Mr Cantor, the House of Representatives majority leader, said he also backed Mr Obama.
The Virginia Republican said: "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."
Mr Obama said that Mr Assad had to be held accountable for the chemical attack and that he was confident Congress would back him.
Jonathan MarcusBBC diplomatic correspondent
Some elements of the Syrian chemical weapons complex may be buried underground but large parts of it can easily be seen on satellite images.
Much of it is reasonably close to populated areas - and this is the problem.
Attacking such sites with regular explosive bombs might well wreak considerable damage but it could also open up chemical weapons stocks to the air, disperse them over a large area, and potentially cause large numbers of civilian casualties
Can US hit Syria's chemical weapons?
He said he was proposing military action that would degrade President Assad's capacity to use chemical weapons "now and in the future".
"What we are envisioning is something limited. It is something proportional," the president said.
"At the same time we have a broader strategy that will allow us to upgrade the capabilities of the opposition."
Later on Tuesday, Secretary of State John Kerry, Defence Secretary Chuck Hagel and the top US military officer, Gen Martin Dempsey, appeared before the Senate Committee on Foreign Relations.
Mr Kerry told the panel that allies of the US such as Israel and Jordan were "one stiff breeze" away from potentially being hurt by any fresh chemical weapons attacks, and that US inaction would only embolden the Syrian president.
"This is not the time for armchair isolationism," Mr Kerry said. "This is not the time to be spectators to slaughter. Neither our country nor out conscience can afford the cost of silence.
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"We have spoken up against unspeakable horror many times in the past. Now we must stand up and act."
But Mr Kerry said again that there would be no American boots on the ground in Syria and that President Obama was "not asking America to go to war".
Mr Hagel said that "the word of the United States must mean something" and echoed Mr Kerry when adding: "A refusal to act would undermine the credibility of America's other security commitments, including the president's commitment to prevent Iran from acquiring a nuclear weapon."
The BBC's Jane O'Brien, in Washington, says Mr Obama still faces a tough task winning the support of the American people.
She says the latest national opinion poll shows public opposition to any involvement in the Syrian conflict is growing, with six out of 10 Americans against missile strikes and lawmakers also divided.
Barack Obama: "This is a limited, proportional step that will send a clear message"
Mr Obama is due to fly to Sweden, ahead of a G20 meeting in Russia later in the week that is sure to be dominated by Syria.
France has strongly backed the US plan for military action.
President Francois Hollande said on Tuesday: "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."
He called for Europe to unite on the issue, but said he would wait for the Congress vote.
If Congress did not support military action France "would not act alone", he said.
UK Prime Minister David Cameron had also backed Mr Obama, but Parliament rejected a resolution on military action.
Ban Ki-moon: "We must put an end to the atrocities"
At the US hearing, Mr Kerry said the possibility of such a defeat in Congress was "too dire" to contemplate.
He also said that if the UN Security Council failed to act in an appropriate way, Americans "shouldn't turn our backs and say there is nothing we can do".
UN Secretary General Ban Ki-moon had earlier said that the organisation's charter permitted military action only in self defence or with the agreement of the Security Council.
Mr Ban said a US military response could create more turmoil, but that if chemical weapons had been used in Syria then the Security Council should unite and take action against what would be "an outrageous war crime".
The US has put the death toll from the attack on the outskirts of Damascus on 21 August at 1,429, including 426 children, though other countries and organisations have given lower figures.
The Syrian government denies any involvement and blames rebels for the attack.
Continue reading the main story
Interactive: Tent city now home to 130,000
In an interview with French newspaper Le Figaro on Monday, President Assad warned that foreign military action could ignite a wider regional conflict.
Earlier, the UN refugee agency said that more than two million Syrians were now registered as refugees, after the total went up by a million in the past six months.
It said in a statement: "Syria is haemorrhaging women, children and men who cross borders often with little more than the clothes on their backs."
Around half of those forced to leave are children, UN agencies estimate, with about three-quarters of them under 11.
As well as those who have left the country, a further 4.25 million have been displaced within Syria, the UNHCR says, meaning that more people from Syria are now forcibly displaced than from other country.
The BBC's Quentin Sommerville meets Syrians sheltering in a slaughterhouse in Lebanon
On Tuesday, Sweden announced it would become the first European country to grant asylum to all Syrian refugees who apply. They will get permanent resident status.
Sweden has taken in 14,700 asylum seekers from Syria since 2012.
The UN says this is the worst refugee crisis for 20 years, with numbers not seen since the 1994 genocide in Rwanda.