DONALD Trump has warned Russia that US missiles will be launched against the Assad regime in Syria after Moscow threatened to shoot down any American missile launched against Syria and target any US plane, warship or submarine behind the strike.

The US President took to Twitter to respond to Moscow’s ramping up of the rhetoric, saying: "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!' You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!"

Earlier, Alexander Zasypkin, the Russian envoy to Beirut, said: “If there is an American strike, then we...will shoot down the missiles and target the positions from where they were launched. In the past few days, we have seen an escalation towards a significant crisis."

General Sir Richard Barrons, a former Commander of the UK’s Joint Forces Command, responding to Mr Zasypkin’s comments, said: “Well, I hope the ambassador's chosen his words very carefully because what he's actually saying is, if the US and allies decide to strike against Syrian chemical weapons and delivery aircraft, not only are they going to try and shoot down the missiles in flight, which they are capable of doing but won't be with total success, but by saying the words 'launch platforms', he's saying they are going to try and sink ships, sink submarines and shoot aircraft out of the sky.

“That's war," he insisted.

READ MORE: US resolution on Syria chemical weapons attacks vetoed by Russia at UN

The ambassador’s comments came as The World Health Organisation[WHO] demanded “immediate unhindered access” to Douma, the suburb of Damascus, where the suspected chemical strike took place on Saturday evening.

On Tuesday night at the United Nations, Russia vetoed a US proposal to set up an investigation into chemical weapons use in Syria. This was the 12th time Russia had used its veto power at the Security Council to block action targeting Syria.

Earlier, the Organisation for the Prevention of Chemical Warfare said it was preparing to send a team to Douma “shortly”.

READ MORE: International community needs to respond after Syria attack, agrees Theresa May

The WHO said an estimated 500 patients went to health facilities with “signs and symptoms consistent with exposure to toxic chemicals” after the suspected attack.

Peter Salama, the organisation’s Deputy Director-General for emergency preparedness and response, said: “WHO demands immediate unhindered access to the area to provide care to those affected, to assess the health impacts, and to deliver a comprehensive public health response.”

It is estimated that as many as 70 people died in the attack on Douma. The regime of Bashir al-Assad has denied responsibility while Moscow insisted no chemical attack took place.

Meanwhile, European air traffic controllers have issued a "rapid alert" for airlines in the Eastern Mediterranean over the possibility of air strikes into Syria during the next 72 hours.

The European Aviation Security Agency's warning of possible launches of air-to-ground strikes or cruise missiles comes as the US, Britain, France and allies weigh up retaliatory measures to the suspected chemical weapon attack.

READ MORE: Foreign Secretary: Russian veto on US resolution on Syria ‘hugely disappointing’

On Monday, Donald Trump spoke of a "forceful" response within 48 hours ie by today.

At Westminster, Theresa May is expected to make a Commons statement on Syria on Monday when MPs return from their Easter recess.

In August 2013, her predecessor David Cameron failed in his bid to get parliamentary backing for military action against the Assad regime following a chemical attack on the Syrian people.

On Tuesday, the Prime Minister spoke to the US President and France's Emmanuel Macron; all agreed the international community needed to respond to the suspected chemical weapon attack. However, reports suggested she wanted more proof about the source and nature of the strike on Douma.

Asked if he wanted to see a parliamentary vote if Britain did take military against the Assad regime, Tory MP John Baron told BBC Radio 4's Today programme: "In large part yes, because we've been here before and many of us believe we need to approach this in a cautious manner because of the risk of escalation."

READ MORE: US resolution on Syria chemical weapons attacks vetoed by Russia at UN

The Essex MP added: "If there is going to be significant military intervention, then Parliament has every right to ask questions particularly given our poor track record on previous interventions across the region."

Labour MP Emma Reynolds said: "There should be a parliamentary vote and indeed an urgent debate on Monday when we come back from recess.

"I feel strongly that the international community cannot stand by and let President Assad use chemical weapons against his own people with impunity and there is a clear objective of any military action and that is to prevent him from using these weapons against this own people, innocent civilians including children to prevent him doing that again."

The Wolverhampton MP said, with hindsight, Labour had been wrong to vote against reacting to the use of chemical weapons in 2013, claiming it had set a "dangerous precedent".