Russia is suspending a communications channel with the United States set up to avoid midair incidents between Russian and U.S. pilots in the skies over the Syria.

The move is in response to a U.S. missile strike on a Syrian airfield carried out on Thursday in retaliation for a chemical attack by the administration of Syrian President Bashar Assad on Tuesday.

“Russia suspends the Memorandum of Understanding on Prevention of Flight Safety Incidents in the course of operations in Syria signed with the US,” the Russian Foreign Ministry said in a statement Friday.

Russia and the United States set up the so-called deconfliction line in October 2015 after Russian air forces intervened in the ongoing Syrian civil war.

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Prior to setting up the line, U.S. aircraft had a few close calls with Russian aircraft, getting as close as a couple of miles from each other.

Since setting up the channel, the U.S. and Russian militaries have communicated on a regular basis.

Recently, U.S. officials have said the line would be even more important as the battlefield constricts, with the Islamic State in Iraq and Syria (ISIS) losing territory and competing rebel and administration forces moving in on its de facto capitol of Raqqa.

The line was used most recently Thursday night, when the United States notified Russia it would attack the Syrian airfield, allowing Russian troops there to move to safety.

The U.S. military, under the orders of President Trump, fired 59 Tomahawk cruise missiles at the airfield on Thursday night, or Friday morning in Syria.

The attack was in response to a sarin gas attack officials say Assad launched that killed more than 70 civilians.

The decision to take military action against Assad is a dramatic escalation in U.S. involvement in the 6-year-old civil war, which the U.S. military has never directly intervened in. It was also a notable departure for Trump, who has said he didn't want to push the U.S. deeper into the conflict.

Russia, which supports Assad, has responded by condemning the bombing as an “act of aggression.”

The Russian Foreign Ministry, too, said it was a “clear act of aggression against a sovereign Syria” based on “totally distorted” facts.

“Actions undertaken by the US today inflict further damage to the Russia-US relations,” it said in its statement. “It is obvious that the cruise missile attack was prepared in advance. Any expert understands that Washington’s decision on air strikes predates the Idlib events, which simply served as a pretext for a show of force.”