Diplomats gather for an emergency United Nations Security Council meeting in response to a suspected chemical weapons attack in Syria at United Nations headquarters in New York, New York, USA, 9 April 2018. (Photo:EPA)

Trump was reacting to a warning from Assad's main ally Russia on April 10 that any US missiles fired at Syria over the deadly assault on a rebel enclave would be shot down and the launch sites targeted.

"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 in a post on Twitter.

In response, Russia's Foreign Ministry said in a Facebook post that "smart missiles should fly towards terrorists, not towards the lawful government", referring to Syrian rebels and Assad respectively.

Foreign Ministry spokeswoman Maria Zakharova said any US missile strike could be an attempt to destroy evidence of the reported chemical weapons attack in the Syrian town of Douma, for which Damascus and Moscow have denied any responsibility.

After the attack, the rebel group holed up in Douma - Jaish al-Islam - finally agreed to withdraw from the town. That sealed a major victory for Assad in Syria's civil war, crushing a protracted rebellion in the eastern Ghouta region near Damascus.

Moscow's threat to down US missiles came from its ambassador to Lebanon, Alexander Zasypkin, who said he was referring to a statement by President Vladimir Putin and the Russian armed forces chief of staff.

Zasypkin also said that any hostilities with Washington should be avoided and Moscow was ready for negotiations.

But his remarks could raise fears of direct conflict for the first time between major powers backing opposing sides in Syria's protracted civil war.

Oil prices hit their highest level in more than three years on April 11 after Trump's threat to unleash missiles, and US stock index futures fell sharply over rising concern about possible Russian-US conflict over Syria.

The Kremlin said earlier on the same day it hoped all sides involved in Syria would avoid doing anything to destabilise an already fragile situation in the Middle East, and made clear it strongly opposed any US strike on its ally.

Trump on April 10 cancelled a planned trip to Latin America later this week to focus on responding to the Syria incident, the White House said.

* Syria's foreign ministry accused the United States on April 11 of using "fabrications and lies" as an excuse to target its territory, state media said.

"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," state news agency SANA cited an official source in the ministry as saying.

* US President Donald Trump on April 11 offered to help boost Russia's economy and sought an end to what he called an "arms race," in an apparent diplomatic overture less than one hour after warning Moscow about pending missile strikes in Syria.

"Our relationship with Russia is worse now than it has ever been, and that includes the Cold War. 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?" Trump wrote on Twitter.

* Separately, some major airlines were re-routing flights on April 11 after Europe's air traffic control agency warned aircraft flying in the eastern Mediterranean to exercise caution due to possible air strikes into Syria.

Eurocontrol said in a notification published on April 10 afternoon that air-to-ground and cruise missiles could be used over the following 72 hours and there was a possibility of intermittent disruption to radio navigation equipment.