US President Barack Obama
US President Barack Obama has said he has not yet decided on a plan for retaliatory action against Syria.
But he said he had concluded Syrian government forces were behind a recent chemical weapons attack near Damascus.
Speaking on US television, Obama said the use of chemical weapons affected US national interests and that sending a "shot across the bows" could have a positive impact on Syria's war.
His comments follow a day of behind-the-scenes wrangling at the UN, reports the BBC.
The UK had been pushing for permanent members of the UN Security Council to adopt a resolution which would have authorised measures to protect civilians in Syria.
But Syrian ally Russia refused to agree to the resolution and the meeting produced no end to the diplomatic stalemate which has long characterised the UN position on Syria.
The US State Department criticised "Russian intransigence" and said it could not allow diplomatic paralysis to serve as a shield for the Syrian leadership.
Russia is sending an anti-submarine ship and a missile cruiser to the eastern Mediterranean.
The ships are being sent to strengthen the navy's presence in the area because of the "well-known situation" there, the Russian news agency Interfax has said.
But another news agency, RIA Novosti, quotes a senior naval command spokesman as saying that this is just a planned rotation, unconnected with Syria.
Critics have questioned what purpose a limited strike on Syria could serve, but Obama told the Public Broadcasting Service (PBS) it would send the government of Bashar al-Assad "a pretty strong signal that it better not [use chemical weapons] again".
The US has yet to produce the intelligence it says shows Assad's government is guilty of using chemical weapons, and UN weapons inspectors are still investigating inside Syria.
The team has just begun a third day of on-site investigations, and UN Secretary General Ban Ki-moon has appealed for it to be "given time to do its job". He said the inspectors would finish their investigations and be out of the area by Saturday morning.
Syria denies using chemical weapons and blames opposition fighters for the attack near Damascus on 21 August, which reportedly killed hundreds of people.
It accused the West of "inventing" excuses to launch a strike.
In a sign of growing fears about an impending attack among Syrians, the Associated Press quoted Lebanese officials as saying at least 6,000 Syrians crossed into Lebanon in a 24-hour period through the main Masnaa crossing - compared to a normal daily tally of between 500 and 1,000 refugees.