WASHINGTON (Reuters) - The United States has made slight adjustments to its military activities in Syria to strengthen protection of American forces following cruise missile strikes last week on a Syrian air base that heightened tensions, U.S. officials told Reuters on Monday.

A U.S. fighter stands near a military vehicle, north of Raqqa city, Syria November 6, 2016. REUTERS/Rodi Said

The officials, citing the need to safeguard operations in Syria, declined to specify exactly what measures the United States has taken after the strikes, which Damascus, Tehran and Moscow have roundly condemned. They spoke on condition of anonymity.

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Asked about the Reuters report, a U.S. military spokesman later told a Pentagon news briefing that the U.S. commander for the campaign has been “calling in the resources that he needs” to protect U.S. forces in the wake of the strikes.

The spokesman, Colonel John Thomas, also said U.S. strikes in Syria had become more defensive and acknowledged the pace had slowed somewhat since last Friday.

“I don’t think that is going to last for very long, but that is up to (Lieutenant General Stephen) Townsend,” Thomas said, stressing there had been no attempts by Syria or its allies to retaliate against U.S. troops so far.

President Donald Trump ordered the cruise missile strike on Syria’s Shayrat air base last week in response to what Washington and its allies say was a poison gas attack by Syria’s military in which scores of civilians died.

It was the first time Washington has directly targeted Syrian President Bashar al-Assad’s government in six years of civil war, and has pushed Trump’s administration into proclaiming that Washington still wants Assad eventually removed from power.

But the volley of Tomahawk missiles has not changed the view held by Damascus and its allies that the United States is no more eager than before to take the sort of strong action needed to defeat him.

White House spokesman Sean Spicer said on Monday the primary U.S. focus remained on defeating Islamic State, which U.S. officials say has used its Syria stronghold of Raqqa as a hub to plot attacks against the West.

Thomas also said the U.S. military’s priority was unchanged.

“(Islamic State) continues to be our main focus,” Thomas said.

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Defense Secretary Jim Mattis said on Monday the cruise missile strikes on Shayrat air base damaged or destroyed a fifth of Syria’s operational aircraft, in what he said was a clear warning to Damascus against any repeat chemical attack.

“The Syrian government would be ill-advised ever again to use chemical weapons,” Mattis said in a statement.

Damascus denies carrying out the chemical attack in the town of Khan Sheikhoun near the Turkish border, which killed at least 87 people, 31 of them children. Moscow called the strike an act of aggression that violated international law.

A joint command center made up of the forces of Russia, Iran and militias supporting Assad has warned it would respond to any new aggression.

Unlike in Iraq, where U.S. forces are battling the Islamic State at the invitation of Iraq’s government, Washington is waging air strikes in Syria against the militants without the permission of Damascus. It also has about 1,000 forces on the ground, mainly advising and training local Kurdish and Arab militia to battle Islamic State.

To avoid accidentally clashing with Russian forces, who are fighting in support of Assad, the United States has had an agreement that allows for the two militaries to communicate.

But Russia’s Deputy U.N. Ambassador Vladimir Safronkov told the U.N. Security Council on Friday Russia’s defense ministry had “stopped its cooperation with the Pentagon” under the agreement.

The U.S. military, which confirmed on Friday morning it believed the line of communications was still active, has since stopped commenting on whether it was operational.

Thomas said the United States was still able to avoid accidentally crossing paths with Russian forces but wouldn’t say how, leaving open the possibility such communication could be minimal, perhaps just pilot-to-pilot talks by radio.

“We have continued to deconflict as necessary with the Russians because whenever we are flying we have to use all the available means to make sure that we don’t have any mid-air incidents,” Thomas said.