(Adds Russian comment on alleged U.S. naval movements, British PM)

* Trump replies to Russian threat to shoot down any missiles

* Trump to Russia: Don’t support “gas-killing animal” Assad

* Russia to Trump: Target “terrorists, not legal government”

* 43 dead from symptoms consistent with toxic chemicals -WHO

* Eurocontrol warns airlines of missile strike risk in Syria

By Susan Heavey, Makini Brice and Tom Perry

WASHINGTON/BEIRUT, April 11 (Reuters) - U.S. President Donald Trump warned Russia on Wednesday 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.

Trump was reacting to a warning from Russia on Tuesday that any U.S. missiles fired at Syria over the deadly assault on a rebel enclave would be shot down and the launch sites targeted.

His comments raised fears of direct conflict over Syria for the first time between the two world powers backing opposing sides in the country’s protracted 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 in a post 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 in a Facebook post that “smart missiles should fly towards terrorists, not towards the lawful government”.

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

Dozens of Douma inhabitants died and hundreds were injured in the attack, according to the World Health Organization (WHO).

After Trump’s tweet, the Syrian Observatory for Human Rights - a British-based war monitor with a network of sources on the ground - reported that pro-government forces were emptying main airports and military air bases.

The Russian military said later it had observed movements of U.S. naval forces in the Gulf. Any U.S. strike would probably involve the navy in waters within range of Syria, given the risk to aircraft from Russian and Syrian air defence systems. A U.S. Navy 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 an official source in the ministry as saying.

TALKING ABOUT TIMING

After the Douma attack, the insurgent group dug in there, Jaish al-Islam, finally agreed to withdraw. That clinched 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 Douma attack and such “a shocking and barbaric act” could not go unchallenged.

“We’re rapidly reaching an understanding of what happened on the ground,” she told reporters. “We’ll be working with our closest allies to consider how we can ensure that those responsible are held to account, and also how we can prevent and deter the humanitarian catastrophe of the use of chemical weapons in the future.”

Oil prices jumped to their highest level in more than three years on Wednesday after Trump’s missile warning, and U.S. stock index futures fell sharply amid alarm over possible Russian-U.S. conflict over Syria.

White House officials did not immediately provide any more detail about Trump’s remarks. The U.S. Defense Department said it did not comment on “potential future military actions”.

Trump’s disclosure of his apparent decision to strike as well as the kind of weaponry to be used in a future military operation is likely to frustrate military planners, who hold such information closely.

Trump had repeatedly said he would not telegraph military moves against foes such as North Korea and Islamic State. On Monday he said he would decide within 48 hours on a forceful answer to the attack in Syria, later telling reporters: “When, I will not say, because I don’t like talking about timing.”

43 DEAD FROM CHEMICAL WEAPONS EXPOSURE -WHO

The World Health Organization said on Wednesday that 43 people had died in Saturday’s attack on Douma from “symptoms consistent with exposure to highly toxic chemicals”, and more than 500 in all had been treated.

The WHO cautioned that it has no formal role in forensic inquiries into the use of chemical weapons. International inspectors are seeking clearance from Damascus to visit Douma under safe conditions to determine whether globally banned munitions were used, though they will not assign blame.

Moscow and Washington blocked attempts by each other at the United Nations Security Council on Tuesday to set up international investigations into chemical weapons attacks in Syria.

Moscow’s threat to down U.S. missiles came from its ambassador to Lebanon, Alexander Zasypkin, who said it was based on previous statements by President Vladimir Putin and the Russian armed forces chief of staff.

The Russian military said on March 13 that it would respond to any U.S. strike on Syria by targeting any missiles and launchers involved. Russia is Assad’s most powerful ally and its devastating air power has helped him wrest back large areas of territory from rebels since 2015.

Zasypkin also said a clash between Russia and the United States over Syria “should be ruled out and therefore we are ready to hold negotiations”.

AIRLINES TOLD TO BEWARE OF POSSIBLE MISSILES

With tensions growing, however, pan-European air traffic control agency Eurocontrol warned airlines to exercise caution in the eastern Mediterranean due to the possible launch of air strikes into Syria over the next 72 hours.

Eurocontrol said that air-to-ground and cruise missiles could be used within that period and there could be intermittent disruptions of radio navigation equipment.

Aviation regulators have been stepping up monitoring of conflict zones since Malaysia Airlines flight MH17 was downed by a surface-to-air missile over Ukraine in 2014, killing all 298 people on board. Recent warnings have tended to be after military action has started, so Eurocontrol’s pre-emptive notice suggests a heightening of regulatory scrutiny.

Both Russia and Iran, Assad’s other main ally, have warned his enemies against military action in recent days, underlining their commitment to the Syrian government they have armed and supported through years of conflict.

Ali Akbar Velayati, the top adviser to Iranian Supreme Leader Ayatollah Ali Khamenei, said during a visit to Damascus on Tuesday that an Israeli attack on an air base in Syria earlier this week would “not remain without response”.

Israel held top-level security consultations on Wednesday amid concern it could be targeted by Syria or Iran if the United States strikes Syrian government forces.

RISK OF “UNCONTROLLABLE ESCALATION”

On Monday, U.N. Syria peace envoy Staffan de Mistura cited the air base strike along with other recent events in Syria in a briefing to the Security Council, cautioning against a “situation of uncontrollable escalation”.

Syria’s Russian-supplied air defences shot down an Israeli F-16 jet in February during a previous bombing run against what Israel described as Iranian-backed positions in Syria.

Last year, the United States carried out strikes from two Navy destroyers against a Syrian air base after another toxic gas attack on a rebel-controlled pocket.