You are not permitted to download, save or email this image. Visit image gallery to purchase the image.
US President Donald Trump warned Russia on Wednesday (local time) of imminent military action in Syria over a suspected poison gas attack, declaring that missiles "will be coming" and lambasting Moscow for standing by Syrian President Bashar al-Assad.
The White House pushed back against suggestions that Trump had broadcast his plans for military strikes via Twitter, saying he had not laid out a timetable for action, that all options were still on the table and he was assessing how to respond.
Trump's tweet was reacting to a warning from Russia that any US missiles fired at Syria over the deadly assault on the rebel enclave of Douma near Damascus would be shot down and the launch sites targeted.
His comments raised the prospect of direct conflict over Syria for the first time between the two world powers backing opposing sides in the seven-year-old civil war, which has aggravated instability across the Middle East.
"Russia vows to shoot down any and all missiles fired at Syria. Get ready Russia, because they will be coming, nice and new and 'smart!'," Trump wrote on Twitter.
"You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!" Trump tweeted, referring to Moscow's alliance with Assad.
In response, Russia's foreign ministry said: "Smart missiles should fly towards terrorists, not towards the lawful government".
Damascus and Moscow have denied any responsibility and say the incident is bogus.
Dozens of people in Douma died and hundreds were injured in the attack, according to the World Health Organisation (WHO).
US Defense Secretary Jim Mattis, striking a cautious tone after Trump's threat of missile strikes, said the United States was assessing intelligence about the suspected attack.
Asked if he had seen enough evidence to blame Assad, Mattis said: "We're still working on this."
The US military was ready to provide military options, if appropriate, he added. It was unclear if his remarks reflected unease about Trump's apparent move toward military action.
Two US government sources told Reuters the United States still did not have 100% solid evidence of what nerve agent was used in Syria and where it came from. However, there is some evidence it was sprayed from helicopters, they said.
In Moscow, the head of a Russian parliamentary defence committee, Vladimir Shamanov, said Russia was in direct contact with the US Joint Chiefs of Staff about the situation.
The Syrian Observatory for Human Rights, a Britain-based war monitor, said pro-government forces were emptying main airports and military air bases. The Syrian military has also been repositioning some air assets to avoid possible missile strikes, US officials told Reuters.
The Russian military said it had observed movements of US Navy forces in the Gulf. Any US strike would probably involve the navy, given the risk to aircraft from Russian and Syrian air defences. A US guided-missile destroyer, the USS Donald Cook, is in the Mediterranean.
The Syrian foreign ministry accused the United States, which has supported some rebel groups in Syria's conflict, of using "fabrications and lies" as an excuse to hit its territory.
"We are not surprised by such a thoughtless escalation by a regime like the United States regime, which sponsored terrorism in Syria and still does," the state news agency SANA cited a ministry official as saying.
After the Douma attack, the insurgent group dug in there, Jaish al-Islam, finally agreed to withdraw. That sealed a huge victory for Assad, crushing a long rebellion in the eastern Ghouta region near the capital Damascus.
In London, British Prime Minister Theresa May said all the indications pointed to Syrian government responsibility for the attack and such "a shocking and barbaric act" could not go unchallenged.
The BBC reported that May was ready to give the go-ahead for Britain to take part in military action. She would not seek approval from parliament, the BBC said, despite calls from the opposition Labour Party for parliament to be given a say.
She also ordered British submarines to move within missile range of Syria in readiness for strikes against the Syrian military that could begin as early as Thursday night, the Daily Telegraph newspaper reported.
Oil prices jumped to their highest level in more than three years on Wednesday after Trump's warning, and US stock index futures fell sharply due to alarm about a possible Russian-US conflict over Syria.