Russia on Friday condemned the U.S. missile strike against Syrian government forces as an attack against its ally, and said it was pulling out of an agreement to minimize the risk of in-flight incidents between U.S. and Russian aircraft operating over Syria.

Even as Russian officials expressed hope that the U.S. strike against Bashad al-Assad would not lead to an irreversible breakdown in relations with Moscow, the Kremlin’s decision to suspend the 2015 memorandum of understanding on the air operations immediately raised tensions in the skies over Syria.

President Vladi­mir Putin’s spokesman said the risk of confrontation between aerial assets of the U.S.-led coalition and Russia has “significantly increased” after President Trump ordered the launch 59 Tomahawk cruise missiles at a Syrian military air base in retaliation for a chemical attack that killed scores of civilians.