UNITED NATIONS/MOSCOW/BEIRUT, April 7 (Reuters) - Russia warned on Friday that U.S. 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.

Carlos Barria / Reuters President Donald Trump delivers an statement about missile strikes on a Syrian airbase, at his Mar-a-Lago estate in West Palm Beach, Florida, U.S., April 6, 2017.

The U.S. 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 U.S. The consequences of this for regional and international stability could be extremely serious,” Russia’s deputy U.N. envoy, Vladimir Safronkov, told a meeting of the U.N. Security Council on Friday.

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

U.S. 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 behavior 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.

TIMOTHY A. CLARY via Getty Images US Ambassador to the UN, Nikki Haley holds photos of victims as she speaks as the UN Security Council meets in an emergency session at the UN on April 5, 2017, about the suspected deadly chemical attack that killed civilians, including children, in Syria.

FURTHER STEPS?

U.S. 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 U.N. 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.”

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

U.S. officials said the intervention was a “one-off” intended to deter future chemical weapons attacks, and not an expansion of the U.S. role in the Syrian war.

The action is likely to be interpreted as a signal to Russia, as well as countries such as North Korea, China and Iran where Trump has faced foreign policy tests early in his presidency, that he is willing to use force.

Senior U.S. 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.

Sana Sana / Reuters Syria's President Bashar al-Assad speaks during an interview with Croatian newspaper Vecernji List in Damascus, Syria, in this handout picture provided by SANA on April 6, 2017.

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 U.S. 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.

U.S. lawmakers from both parties on Friday backed Trump’s action but demanded he spell out a broader strategy for dealing with the conflict and consult with Congress on any further action.

The U.N. Security Council had been negotiating a resolution, proposed by the United States, France and Britain on Tuesday, to condemn the gas attack and push the Syrian government to cooperate with international investigators.

Russia said the text was unacceptable and after the council failed to bridge the gap in closed-door negotiations on Thursday diplomats said it was unlikely to be put to a vote.

Russia expects U.S. Secretary of State Rex Tillerson to explain Washington’s stance in light of the missile strikes when he visits Moscow in the coming week, Interfax news agency cited a Russian Foreign Ministry spokeswoman as saying.

Handout . / Reuters Battle damage assessment image of Shayrat Airfield, Syria, is seen in this DigitalGlobe satellite image, released by the Pentagon following U.S. Tomahawk Land Attack Missile strikes from Arleigh Burke-class guided-missile destroyers, the USS Ross and USS Porter on April 7, 2017.

Washington has long backed rebels fighting Assad in a multi-sided civil war that has killed more than 400,000 people and driven half of Syrians from their homes since 2011.

The United States has conducted air strikes against Islamic State, which controls territory in eastern and northern Syria, and a small number of U.S. troops are helping militias fighting its militants.

Asked whether the strikes set back any efforts to work with Russia to defeat Islamic State, sometimes known as ISIS, White House spokesman Sean Spicer said:

“There can be a shared commitment to defeat ISIS and also agree that you can’t gas your own people.”

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

Tuesday’s attack was the first time since 2013 that Syria was accused of using sarin, a banned nerve agent it was meant to have given up under a Russian-brokered, U.N.-enforced deal that persuaded Obama to call off air strikes four years ago.

Video depicted limp bodies and children choking while rescuers tried to wash off the poison gas. Russian state television blamed rebels and did not show footage of victims.

The U.S. strikes cheered Assad’s enemies, after months when Western powers appeared to grow increasingly resigned to his staying in power. But opposition figures said an isolated assault was far from the decisive intervention they seek.

Neither the Trump administration nor its predecessor has laid out a policy aimed at ending theSyrian conflict.

“The big question for all those who are engaged in military action in Syria is what is their plan to stop the killing and bring a durable peace that can deliver a modicum of hope to the people ofSyria?” David Miliband, head of the International Rescue Committee humanitarian agency, told Reuters Television.

(Additional reporting by Phil Stewart, Idrees Ali, Roberta Rampton and Patricia Zengerle in Washington, Steve Holland in Florida, Roselle Chen in New York and Denis Pinchuk in Moscow; Writing by Alistair Bell; Editing by James Dalgleish)