(Adds May to hold Cabinet meeting, U.N. Security Council to discuss Syria)
* 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
* Britain ready to join action
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.
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 U.S. missiles fired at Syria over the deadly assault on Saturday 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 Organization (WHO).
U.S. 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 U.S. 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 U.S. government sources told Reuters the United States still did not have 100 percent 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 U.S. 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, U.S. officials told Reuters.
The Russian military said it had observed movements of U.S. Navy forces in the Gulf. Any U.S. strike would probably involve the navy, given the risk to aircraft from Russian and Syrian air defences. A U.S. 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.
BRITAIN READY
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.
May has called ministers to a Thursday cabinet meeting on Syria, which media reports said was likely to lead to London joining in a military response to the suspected chemical attack.
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 U.S. stock index futures fell sharply due to alarm about a possible Russian-U.S. conflict over Syria.
HIGHLY TOXIC
The WHO said 43 people had died in Saturday's attack on Douma from "symptoms consistent with exposure to highly toxic chemicals", and more than 500 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.
The Security Council is scheduled to meet on Thursday at the request of Bolivia, which wants to discuss the escalation of rhetoric over Syria and threats of unilateral military action.
U.N. Secretary-General Antonio Guterres said he called the ambassadors of the Council's veto-wielding powers, the United States, Britain, France, Russia and China, "to reiterate my deep concern about the risks of the current impasse and stressed the need to avoid the situation spiraling out of control."
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".
Putin spoke to Israeli Prime Minister Benjamin Netanyahu on Wednesday and urged him to do nothing to destabilise Syria.
AIRLINES BEWARE
With tensions growing, however, 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.
Both Russia and Iran, Assad's other main ally, have in recent days warned his enemies against military action, 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 on Monday would "not remain without response".
Israel held top-level security consultations on Wednesday on the possibility it could be targeted by Syria or Iran if the United States strikes Syrian government forces.
Syria's Russian-supplied air defences shot down an Israeli F-16 jet in February during a 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.
(Additional reporting by Dahlia Nehme in Beirut, Jeffrey Heller in Jerusalem, Michelle Nichols at the United Nations, Andrew Osborn and Maria Kiselyova in Moscow, William Schomberg in London, Anthony Deutsch in Amsterdam, Steve Holland, Idrees Ali, Mark Hosenball, Phil Stewart, Jeff Mason and Yara Bayoumy in Washington, Jamie Freed in Singapore and Stephanie Nebehay in Geneva; Writing by Mark Heinrich, Editing by Angus MacSwan, Alistair Bell, Toni Reinhold)
