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.
‘No blind eye’
President Obama said the military operation could happen tomorrow, next week or in the near future.
“We cannot and will not turn a blind eye to what happened in Damascus,” he said.
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 president’s role as commander-in-chief was “always strengthened when he enjoys the expressed support of the Congress”.
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.
Last week, British MPs defeated a government motion to take part in any military action in Syria.
On Saturday, UK Prime Minister David Cameron tweeted: “I understand and support Barack Obama’s position”
Earlier on Saturday, Russian President Vladimir Putin challenged the US to present to the UN evidence that Syria 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”.
Forces which could be used against Syria:
• Four US destroyers – USS Gravely, USS Ramage, USS Barry and USS Mahan – are in the eastern Mediterranean, equipped with cruise missiles. The missiles can also be fired from submarines, but the US Navy does not reveal their locations
• Airbases at Incirlik and Izmir in Turkey, and in Jordan, could be used to carry out strikes
• Two aircraft carriers – USS Nimitz and USS Harry S Truman are in the wider region
• French aircraft carrier Charles de Gaulle is currently in Toulon in the western Mediterranean
• French Rafale and Mirage aircraft can also operate from Al-Dhahra airbase in the UAE