US President Barack Obama says he is to seek Congressional authorisation for military operations against Syria.
Mr Obama said the US should take action in Syria, but it was important to have a debate and vote in Congress, which reconvenes on 9 September.
The US says its intelligence reports indicate the Syrian government carried out chemical weapons attacks on 21 August in which 1,429 people died.
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.
Mr Obama said the operation would be limited in duration and strong to deter future chemical attacks.
He said the military operation could happen tomorrow, next week or in the near future.