By Michelle Nichols, Andrew Osborn and Tom Perry

UNITED NATIONS/MOSCOW/BEIRUT (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 at the Shayrat air base, which the Pentagon says was involved in a chemical weapons attack this week. (Graphic locator map on attack - http://tmsnrt.rs/2nm68H0)

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. Syria denies it carried out the attack.

They catapulted Washington into confrontation with Russia, which has 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. (Graphic explaining kind of missile used in attack - http://tmsnrt.rs/2nkJYWa)

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, was hosting Chinese President Xi Jinping at his Mar-a-Lago resort in Florida on Thursday night when the attack occurred.

Story continues

Treasury Secretary Steve Mnuchin, in Florida with Trump, said on Friday the United States would announce additional sanctions on Syria in the near future but offered no specifics.

Russia's Defense Ministry responded to the attack by calling in the U.S. military attache in Moscow to say that at midnight Moscow time (5 p.m. EDT) it would close down a communications line used to avoid accidental clashes between Russian and U.S. forces in Syria, Interfax new agency said. U.S. warplanes frequently attack Islamic State militants in Syria and come close to Russian forces.

"PREPARED TO DO MORE"

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."

Secretary of State Rex Tillerson, who also was in Florida with Trump and is scheduled to go to Moscow next week, said he was disappointed but not surprised by the Russian reaction because it showed continued support for Assad.

Iran, which supports Assad and has been criticized by Trump, condemned the strike, with President Hassan Rouhani saying it would bring "only destruction and danger to the region and the globe."

U.S. officials called the intervention a "one-off" intended to deter future chemical weapons attacks and not an expansion of the U.S. role in the Syrian war.

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

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

The United States is now likely to be more aggressive in pursuing intelligence about Syria's suspected chemical weapons program. The Pentagon has also signaled interest in determining any Russian complicity.

"At a minimum, the Russians failed to rein in the Syrian regime activity," a senior U.S. military official told reporters, speaking on condition of anonymity.

The official also said the United States had been unable to determine if a Russian or Syrian aircraft bombed a hospital that was treating victims of the chemical attack.

Russia joined the war on Assad's behalf in 2015, turning the momentum in his favor. Although Moscow supports opposing sides in the war between Assad and rebels, the United States and Russia say they share a single main enemy, Islamic State.

Tillerson said the strike took out about 20 percent of the seventh wing of the Syrian air force and hit a fueling facility. The base's runway was still in use.

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 U.S. attack killed six people and called it "blatant aggression" that 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 diplomats said it was unlikely to be put to a vote.

MOSCOW WANTS EXPLANATION

Russia expects Tillerson to explain Washington's stance when he visits Moscow next week, Interfax news agency cited a Russian Foreign Ministry spokeswoman as saying.

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 rebel militias.

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.”

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. (Chart of cumulated reported deaths from chemical gases in Syria - http://tmsnrt.rs/2o5Xbz6)

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 the Syrian conflict.

(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 and Bill Trott; Editing by James Dalgleish and Lisa Shumaker)