UNITED NATIONS/MOSCOW - Russia warned on Friday that US cruise missile strikes on a Syrian air base could have "extremely serious" consequences, as President Donald Trump's first major foray into a foreign conflict opened up a rift between Moscow and Washington.

The warships USS Porter and USS Ross in the Mediterranean Sea launched dozens of Tomahawk missiles that hit the airstrip, aircraft and fuel stations of Shayrat air base, which the Pentagon says was involved in a chemical weapons attack this week.

It was Trump's biggest foreign policy decision since taking office in January and the kind of direct intervention in Syria's six-year-old civil war his predecessor Barack Obama avoided.

The strikes were in reaction to what Washington says was a poison gas attack by the government of Syrian President Bashar al-Assad that killed at least 70 people in rebel-held territory.

The US action catapulted Washington into confrontation with Russia, which has military advisers on the ground aiding its close ally Assad.

"We strongly condemn the illegitimate actions by the US. The consequences of this for regional and international stability could be extremely serious,” Russia's deputy UN envoy, Vladimir Safronkov, told a meeting of the UN Security Council on Friday.

Russian Prime Minister Dmitry Medvedev charged that the US strikes were one step away from clashing with Russia's military.

US officials informed Russian forces ahead of the missile strikes, and avoided hitting Russian personnel. Satellite imagery suggests the base houses Russian special forces and helicopters, part of the Kremlin's effort to help Assad fight Islamic State and other militant groups.

Trump has frequently urged improved relations with Russia, strained under Obama over Syria, Ukraine and other issues, but he said action had to be taken against Assad.

"Years of previous attempts at changing Assad’s behaviour have all failed and failed very dramatically," Trump said as he announced the attack on Thursday night from his Florida resort, Mar-a-Lago, where he was meeting Chinese President Xi Jinping.

US Ambassador to the United Nations Nikki Haley said on Friday the Trump administration was ready to take further steps if needed. "We are prepared to do more, but we hope that will not be necessary," she told the UN Security Council. "The United States will not stand by when chemical weapons are used. It is in our vital national security interest to prevent the spread and use of chemical weapons."

US allies from Asia, Europe and the Middle East expressed support, if sometimes cautiously.

Senior US military officials said the missiles destroyed up to 20 Syrian aircraft and damaged fuel sites and a surface-to-air missile system. Assad's office said Syria would strike its enemies harder.

Damascus and Moscow denied Syrian forces were behind the gas attack but Western countries dismissed their explanation that chemicals leaked from a rebel weapons depot after an air strike.

The Syrian army said the US attack killed six people and called it "blatant aggression" which made the United States a partner of "terrorist groups" including Islamic State. There was no independent confirmation of civilian casualties.

Russia's Defence Ministry notified the Pentagon it would close down on Friday communications used to avoid accidental clashes in Syria, Interfax news agency said. Russia joined the war on Assad's behalf in 2015, turning the momentum of the conflict in his favour. Although they support opposing sides in the war between Assad and rebels, Washington and Moscow say they share a single main enemy, Islamic State.

The Pentagon urged Russia to keep military channels open after Moscow said it was going to hang up a vital hotline established to avoid mishaps between the two powers in Syria.

The so-called deconfliction line has been a lifesaving - albeit imperfect - tool since it was set up soon after Russia entered Syria's civil war in late 2015 to prop up President Bashar al-Assad.

Even though the US military used the line to warn Russia of the impending missile strike on an Assad air base near Homs early Friday, a furious Moscow reacted to the attack by its ally by saying it would no longer cooperate with the Americans.

"The Russian side is halting the effect of the memorandum for prevention of incidents and ensuring safety of air flights during operations in Syria which was agreed with the US," the Russian foreign ministry said.

Major Adrian Rankine-Galloway, a Pentagon spokesman, said he hoped Russia would reconsider.

"The Department of Defence maintains the desire for dialogue through the flight safety channel," he said.

"It is to the benefit of all parties operating in the air over Syria to avoid accidents and miscalculation, and we hope the Russian Ministry of Defense comes to this conclusion as well."

A senior US military official said the deconfliction line remained open as of noon Friday and had been used by US and Russian officials since the US strike on a Syrian regime airfield.

"There is no variation of the deconfliction line or deconfliction coordination with the Russians," the official said.

The hotline was established between US officers monitoring the war from an operations center at a base in Qatar and their Russian counterparts operating in Syria.

The link is a regular phone line staffed on the US side by a Russian-speaking colonel and has been used daily since its inception.

Moscow's move to abandon the hotline could dramatically raise the risk to pilots and ground forces on all sides.

It was used in February to stop a Russian strike on US-backed fighters in several small villages in northern Syria after the Russian pilots apparently mistakenly thought Islamic State group forces were in the area.