President Barack Obama says the US should take military action against Syria and he will seek congressional authorisation for intervention.
The US says the Syrian government carried out chemical weapons attacks on 21 August in which 1,429 people died.
Mr Obama said the operation would be limited in duration and strong to deter future chemical attacks. Congress is due to reconvene on 9 September.
The Syrian government denies it was behind the attacks and blames rebels.
UN inspectors who have been investigating the attacks arrived in the Netherlands on Saturday with samples from site visits, which will be tested in laboratories in Europe.
The Organisation for the Prohibition of Chemical Weapons, which is overseeing the investigation, said the whole testing process could take up to three weeks, although "every effort" would be made to expedite it.
Senior White House officials told the BBC's Katty Kay that Mr Obama's decision to seek congressional approval was made by the president on Friday afternoon. It had not been planned until then.
The officials added that they believed they would get congressional approval, although they were aware of the risks, our correspondent adds.
Later on Saturday, Mr Obama formally asked Congress to authorise military action.
The draft legislation seeks approval for the use of force to prevent the Syrian government from carrying out chemical weapons attacks.
'No blind eye'
President Obama said the US was prepared to strike whenever it chose. "Our capacity to execute this mission is not time-sensitive."
He added: "We cannot and will not turn a blind eye to what happened in Damascus."
As commander-in-chief, Mr Obama has the constitutional authority to order military action without the backing of Congress.
However, he said it was important to have the debate.
Senate Republican Leader Mitch McConnell welcomed the announcement, saying the president's role as commander-in-chief was "always strengthened when he enjoys the expressed support of the Congress".
Republican Senators John McCain and Lindsey Graham, who have been pushing for US intervention in Syria, also gave their backing to the vote.
However, they warned against limited strikes which would not change the balance of the conflict, calling the prospect "an inadequate response to the crimes against humanity that [Syrian President] Assad and his forces are committing".
The BBC's Katy Watson in Washington says that if Mr Obama is to intervene, he wants the people - and politicians - on his side.
But what is unclear is what action he would take if Congress votes against involvement, our correspondent says.
On Thursday, British MPs defeated a government motion to take part in any military action in Syria.
After the president's address UK Prime Minister David Cameron tweeted: "I understand and support Barack Obama's position".
Mr Obama did not speak to Mr Cameron before his statement but did call President Hollande of France, the White House said.
France has also backed military action in Syria. The French parliament is due to reconvene next week.
France will wait for discussions in the US Congress and French parliament before making a decision on military intervention, a French official told the Associated Press.
Earlier on Saturday, Russian President Vladimir Putin challenged the US to present to the UN evidence that Syria had attacked rebels with chemical weapons.
Mr Putin said it would be "utter nonsense" for Syria's government to provoke opponents with such attacks.
Russia - a key ally of Syria - has previously warned that "any unilateral military action bypassing the UN Security Council" would be a "direct violation of international law".
Moscow, along with China, has vetoed two previous draft resolutions on Syria.
The BBC's Jeremy Bowen in Damascus says people there are worried and are making preparations.
They do not know what Mr Obama meant by a limited attack and what consequences it will have, he adds.
The main findings of the unclassified US evidence state that:
- the attacks killed 1,429 people, including 426 children
- Syrian military chemical weapons personnel were operating in the area in the three days before the attack
- Satellite evidence shows rockets launched from government-held areas 90 minutes before the first report of chemical attack
- 100 videos attributed to the attack show symptoms consistent with exposure to a nerve agent
- Communications were intercepted involving a senior Damascus official who "confirmed chemical weapons were used" and was concerned about UN inspectors obtaining evidence
Syrian President Bashar al-Assad has said his country will defend itself against any Western "aggression".