US President Donald Trump told Russia in an early morning tweet to prepare for "nice and new and 'smart!'" missile attacks in Syria after an alleged gas attack on the last rebel stronghold in the country.

The tweet came after the Russian ambassador to Lebanon said in an interview on Tuesday with Hezbollah-owned al-Manar TV that "if there is a US missile attack, we - in line with both Putin and Russia's chief of staff's remarks - will shoot down US rockets and even the sources that launched the missiles".

Trump further chided Russia for its support of Syrian President Bashar al-Assad in Wednesday's tweet, saying it "shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!"The warnings come as the United States and several European countries threatened to use military action against the Syrian government and its main ally, Russia, in response to a suspected chemical weapons attack on the rebel-held town of Douma.

Saturday's attack in Eastern Ghouta has killed dozens of people, mostly women and children, according to activists and local medics.

The Syrian government and Russia have denied that a chemical attack took place.

On Tuesday, rival draft resolutions by the US and Russia to set up a new expert body to probe chemical weapons attacks in Syria both failed to pass at the United Nations Security Council.

James Mattis, US defence secretary, did not rule out any military action against Assad's forces, while US ambassador to the UN, Nikki Haley, warned Washington was ready to "respond" to the attack regardless of whether the Security Council acted or not.Vladimir Shamanov, chairman of the defence affairs committee in Russia's lower house of parliament, said on Tuesday that "Russia has unlimited opportunities in retaliation, but I would like to avoid it", according to Russian-language reports.

“Smart missiles should fly towards terrorists, not [Syria’s] lawful government, which has spent several years fighting against international terrorism on its territory,” Russian Foreign Ministry spokesperson Maria Zakharova said in response to Trump's tweet.

Meanwhile, the Syrian government has put its forces on "high alert" amid the looming threat of a US military response.

With Russian military assistance, President Assad launched a bloody offensive on Eastern Ghouta, which had been under rebel control since mid-2013.

Since the start of the aerial bombardment campaign on February 18, the offensive has claimed more than 1,600 civilian lives, and has, through a series of deals reached with rebel groups, internally displaced more than 45,000 people according to the UN.

Trump later tweeted that Russian-US relations are "worse now" than ever before.

The president concluded by opening the possibility of a detente: "There is no reason for this. Russia needs us to help with their economy, something that would be very easy to do, and we need all nations to work together. Stop the arms race?"Al Jazeera.