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 signalled 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.
‘Broader strategy’
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”
Mr Boehner signalled his support for Mr Obama’s call for action, saying 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.”
Ms Pelosi said she did not believe Congress would reject a resolution calling for force.
Mr Obama said that Mr Assad had to be held accountable for the chemical attack and that he was confident Congress would back him.
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..
Secretary of State John Kerry, Defence Secretary Chuck Hagel and the top US military officer, Gen Martin Dempsey, are appearing before the Senate Committee on Foreign Relations.
Mr Kerry told the panel that US allies 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.
“We have spoken up against unspeakable horror many times in the past. Now we must stand up and act.”
There will also be a classified briefing for all members of Congress.
Mr Obama will head to Sweden late on Tuesday for a G20 meeting 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.
Francois Hollande: “We are taking all necessary measures to confront this threat”
On Tuesday, UN Secretary General Ban Ki-moon 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.
In an interview with French newspaper Le Figaro on Monday, President Assad said such an action would have been “illogical”.
He warned that foreign military action could ignite a wider regional conflict.
As tensions rose on Tuesday, a senior Israeli defence official confirmed to the BBC that a missile had been fired to test its defence systems.
The BBC’s Richard Galpin in Jerusalem says it is a sign that Israel is taking very seriously the possibility that any US air strikes could lead to retaliatory attacks on Israel – either by Syria itself or by its ally, the Shia militia Hezbollah in Lebanon.
‘Overwhelming burden’
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.
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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.
Pointing out that more than 97% of Syria’s refugees are being hosted by countries in the surrounding region, the UNHCR said the influx was “placing an overwhelming burden on their infrastructures, economies and societies”.
Jeremy Bowen reports from one flat where 42 people are sheltering
It appealed again for “massive international support”.
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.