President Barack Obama says he will seek congressional authorization for taking military action against Syria.
The US says the Syrian government carried out chemical weapons attacks on August 21 in which 1,429 people died.
Barack Obama said the operation would be limited in duration and strong to deter future chemical attacks. Congress is due to re-open on September 9.
The Syrian government denies it was behind the attacks and blames rebels.
UN inspectors have now left Syria with samples from site visits, which will go to laboratories in Europe for testing.
President Barack 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, Barack Obama has the constitutional authority to order military action without the backing of Congress.
However, he said it was important to have the debate.
Last week, British MPs defeated a government motion to take military action in Syria.
Russian President Vladimir Putin has challenged the US to present to the UN evidence that Syria attacked rebels with chemical weapons.
Vladimir 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.