WASHINGTON — U.S. warships in the eastern Mediterranean have launched a fiery barrage of Tomahawk missiles at military targets in Syria to punish the Russian-backed government in Damascus for its alleged use of poison gas against civilians outside the capital last weekend, President Donald Trump announced.

Trump authorized the punitive attack against President Bashar Assad’s government and sought to cripple its chemical weapons facilities with what he called precision airstrikes. French and British forces joined the attack, Trump said in a televised address Friday night.

“We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents,” Trump said.

He promised that the United States would not maintain an indefinite presence in the war-torn region, but called on Russia and Iran, which he said are the chief enablers of the Syrian government, to relinquish their support.

“What kind of a nation wants to be associated with the mass murder of innocent men, women and children?” he said.

The predawn air assault Saturday raised concerns of a direct confrontation with Russia, which has an extensive network of ground-to-air missiles in Syria, as well as hundreds of troops and warplanes, and threatened to shoot down any U.S. missiles or planes that it saw as a threat.

The Pentagon’s Central Command, which oversees U.S. military operations in the Middle East, made heavy use of a special communication channel with Russia’s armed forces, called a “deconfliction line,” in recent days to gain a better picture of where Russian forces are deployed in Syria and to reassure Moscow that any U.S. strike will target only Syrian military units, facilities and equipment involved in last Saturday’s attack.

Leaders in Britain, France and Saudi Arabia had pledged to support a strong response, and French warships armed with cruise missiles were near the Syrian coast. The Navy had at least two guided-missile destroyers in the eastern Mediterranean.

Trump’s Twitter posts and comments this week gave Syria time to move aircraft and troops out of likely target areas, and provided Moscow advance warning, making it more likely its advanced air defense batteries could succeed in shooting down U.S. cruise missiles or warplanes, complicating the Pentagon’s task of preparing a response.

It wasn’t clear whether the initial salvos presaged a broader, multiday air campaign against Syrian military command-and-control facilities, and their elaborate network of ground-to-air missile batteries, to clear the skies for allied bombers and other warplanes.

Casualties weren’t yet known. Syrian government troops had evacuated airports and primary military air bases in government-held areas in anticipation of a U.S. air attack, and reportedly moved some Syrian warplanes to Russian-controlled airfields for protection.

Fact-finding teams from the Organization for the Prohibition of Chemical Weapons, an international watchdog agency based in The Hague, were expected to arrive in Douma on Saturday to collect evidence on the April 7 attack that left 43 people dead and wounded hundreds more.

U.S. officials said Syrian helicopters dropped gas-filled barrel bombs in Douma, a suburb east of Damascus. Photos and videos showed victims, including children, foaming at the mouth, choking and twitching in agony.

Local medics and rescue workers said some of the victims emitted an odor that suggested chlorine gas had been used. Others showed symptoms, including constriction of the pupils and convulsions, that suggested an illegal nerve agent such as sarin was mixed in.

The rebel-held town fell to Syrian forces after last week’s bombardment, and Russian troops also have entered the area. Thousands of rebels and civilian residents have been evacuated.

Trump had telegraphed the attack early Wednesday in a tweet that taunted Russia as well as Assad.

“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!’ You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!” Trump wrote.

Trump’s tweet followed a midlevel Russian diplomat’s claim that Russian President Vladimir Putin had authorized shooting down any U.S. missiles aimed at Syria. The Kremlin did not confirm the warning.

“Russia will execute the statement of its president related to any U.S. aggression against Syria, knocking down American missiles and striking at the sources of fire,” Alexander Zasypkin, Russia’s ambassador to Lebanon, told the Lebanese television channel Al-Manar.

Russia has disputed claims by the U.S. and its allies that Assad’s forces used lethal chemical agents against civilians in Douma.

The airstrikes come almost exactly a year after U.S. warships fired 59 Tomahawk cruise missiles at a Syrian air base that U.S. intelligence said was used to carry out a nerve gas attack that killed about 75 people in the town of Khan Sheikhoun.

In that attack, the Pentagon avoided hitting suspected chemical-agent storage facilities at the Shayrat air base, for fear of spreading a toxic cloud, targeting planes and hangars there instead. The Pentagon also gave advance notice to Moscow to ensure any Russian personnel at the base could leave before the attack.

The airfield was back in operation soon after, and Assad’s government began using less-lethal chlorine gas against rebel positions.

Before the latest U.S. attack, the Pentagon had raised concerns that it might not have legal justification for an assault because it hasn’t confirmed that Syrian forces used a banned nerve agent.

Use of any lethal chemical agent as a weapon, especially against civilians, is barred under international law, but unlike sarin, chlorine gas is not specifically prohibited by international treaty.

Forensic evidence from alleged victims of the Douma attack, including blood and urine samples, which the U.S. has received through intermediaries, indicate the presence of chlorine gas, but evidence that a deadly nerve agent such as sarin also was used is less clear, two officials said on condition of anonymity to discuss internal assessments.

Russia has repeatedly denied that a poison gas attack occurred, saying that gruesome photographs of victims were fake.

On Friday, Russian Foreign Minister Sergei Lavrov went much further, telling a state-run news site that Moscow had “irrefutable evidence” that the gas attack was a “performance” staged by a foreign spy service. He did not identify the country or show any proof for his claim.

But Maj. Gen. Igor Konashenkov, a spokesman for the Russian Defense Ministry, blamed Britain, telling reporters that London had “direct involvement,” according to Russia’s state news agency.

Konashenkov also said a Syrian medic who claimed to work at Douma’s central hospital said the victims suffered from smoke inhalation, not chemical exposure. He said a “rent-a-mob” had entered the hospital saying there was a chemical weapons attack and had begun washing one another in front of cameras. The medic said he “had not seen a single patient with signs of poisoning with chemicals,” Konashenkov said.

The British Foreign Office dismissed Moscow’s charges as “the latest in a number of ludicrous allegations from Russia, who have also said that no attack ever happened. This simply shows their desperation to pin the blame on anyone but their client: the Assad regime.”

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