US President Donald Trump has promised quick, forceful action in response to a deadly suspected chemical weapons attack in Syria, appearing to suggest a potential military response.
Trump told a meeting with military leaders and national security advisers he would make a decision by Monday night "or very shortly thereafter" on a response, adding that the US had "a lot of options militarily" on Syria.
"But we can't let atrocities like we all witnessed ... we can't let that happen in our world ... especially when we're able to because of the power of the United States, the power of our country, we're able to stop it."
The suspected chemical weapons attack late on Saturday killed at least 60 people, with more than 1000 injured at several sites in Douma, a city near the capital, Damascus, according to a Syrian aid organisation.
Initial US assessments have been unable to determine conclusively what materials were used in the attack and could not say with certainty that Syrian President Bashar al-Assad's government forces were behind it.
Trump said, however, that Washington was "getting more clarity" on who was responsible for the attack.
US officials told Reuters that Washington was weighing a multinational military response.
Trump and French President Emmanuel Macron spoke by telephone to coordinate their response, seeking a "strong reaction" from the international community, Macron's office said.
On Sunday, Trump, criticised Russian President Vladimir Putin by name on Twitter as he castigated Russia and Iran for backing "Animal Assad."
Moscow said it warned the US of "grave repercussions" if it carried out an attack against Syrian government forces.
The Syrian government and its ally Russia have denied involvement in the attack.
International bodies led by the Organisation for the Prohibition of Chemical Weapons (OPCW) were trying to establish exactly what happened in Douma, a rebel-held town in the eastern Ghouta district.
Syrian government forces had launched an air and ground assault on Douma, the last town held by rebels in eastern Ghouta, on Friday.
The Russian military said on Monday its medics had examined patients in a hospital in Douma and had found no traces of a chemical attack, Interfax news agency reported.
The Syria conflict was further complicated on Monday when unidentified war planes struck a Syrian air base near Homs, killing at least 14 people, including Iranian personnel. Syria and Russia accused Israel of carrying out the attack.
Israel, which has struck Syrian army locations many times in the course of its neighbour's civil war, has neither confirmed nor denied mounting the raid.
But Israeli officials said the Tiyas, or T-4, air base was being used by troops from Iran and that Israel would not accept such a presence in Syria of its arch foe.
Russian Foreign Minister Sergei Lavrov said the strike on the T-4 base was a dangerous development.