Trump muddies threats to bomb Syria, as Russia says it is using crisis line

US President Donald Trump muddied his own threats that air strikes were "coming" in Syria, saying on Thursday that an attack "could be very soon or not so soon at all".

"Never said when an attack on Syria would take place," said Trump on Twitter.

"Could be very soon or not so soon at all! In any event, the United States, under my administration, has done a great job of ridding the region of ISIS. Where is our 'Thank you America?'”

Later on Thursday, the White House said in a statement that "no final decision has been made" on Syria after a meeting between Trump and his national security team.

The line is used and it is active. In general the line is used by both sides

- Dmitry Peskov, Kremlin spokesman

The US president's tweets came a day after he told Russia and "Gas Killing Animal" Bashar al-Assad to "get ready" for "nice, new and smart" missiles from the United Strates.

Trump's latest tweets came hours after the Kremlin said a crisis communications link is being used by both sides to avoid the possibility of clashes over Syria.

Tensions have been mounting over possible US air attacks on the country after a suspected chemical attack on Saturday on Douma, Eastern Ghouta, killed scores of people and seriously affected hundreds more.

US officials on Thursday obtained blood and urine samples from victims of the alleged chemical attack in Syria last weekend and the samples tested positive for chemicals, mainly for chlorine and some for a nerve agent, MSNBC reported.

British ministers were also due to discuss whether to join the US and France in military action on Thursday, while Russian military officials said Douma was now fully in the hands of government forces.

Kremlin spokesman Dmitry Peskov said: "The line is used and it is active. In general the line is used by both sides.

"We continue to consider it extremely important to avoid any steps that could lead to more tension in Syria. We believe that would have an extremely destructive impact on the whole Syria settlement process."

The "deconfliction" line was set up as American and Russian forces increased their presence in Syria during the fight against the Islamic State group.

It was in use last April when the US bombed a Syrian airbase suspected of being the launchpad for a gas attack on the town of Khan Sheikhoun.

France's Emmanuel Macron also said on Thursday he had "proof" that the Syrian government had used chemical weapons, and would respond to it "at a time of our choosing".

However, the French president has also insisted he did "not want an escalation" and that any response would focus on Syria's chemical capabilities, not on allies of the government.

Assad said on Thursday that any action by Western states against his "victory" in Douma would cause greater instability in the region.

"With every victory achieved on the field, the voices of some Western states are raised and actions are intensified in an attempt by them to change the course of events," he said on state television.

"These voices and any possible action will contribute nothing but an increase in instability in the region, threatening international peace and security."