Russian Prime Minister Dmitry Medvedev complained on Friday that U.S. cruise missile strikes on a Syrian airbase were one step away from clashing with the Russian military.
U.S. officials informed Russian forces ahead of the strikes - intended to punish the Syrian government for what they say was a chemical weapons attack earlier this week - and avoided hitting Russian personnel.
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Satellite imagery suggests the Shayrat air base that was struck is home to Russian special forces and military helicopters, part of the Kremlin's effort to help the Syrian government fight Islamic State and other militant groups.
Medvedev, writing on social media, said the U.S. strikes were illegal and had been "one step away from military clashes with Russia."
World leaders rallied around the United States after it launched a missile strike early Friday on a Syrian air base in response to this week's chemical attack, while Iran and Russia condemned the move as "aggression" and suspended crucial coordination with Washington in Syria's congested skies.
The overnight missile attack, which marked the first time the U.S. has directly targeted Syrian President Bashar Assad's forces, was condemned by his allies in Russia and Iran but welcomed by the Syrian opposition and its supporters, who expressed hope it signaled a turning point in the devastating six-year-old civil war.
Russian President Vladimir Putin called the strikes an "aggression against a sovereign nation" which violated international law. Saying the attack was carried out on an "invented pretext," a Kremlin spokesman added that the strikes do significant damage to U.S.-Russian relations.
Russia also said it is suspending its agreement to communicate with the U.S. on the use of Syrian airspace.
The spokesman added that no Russians were hurt in the attack.
Russia also backed Syria's claim that its army is not in possession of any chemical weapons.
The U.S. military gave Russian forces advanced notice of its strikes and did not hit sections of the base where the Russians were believed to be present, Pentagon spokesman Captain Jeff Davis has said.
Davis, briefing reporters on the operation, said the U.S. military had "multiple" conversations with Russian forces on Thursday before the strike, using a line of communication that had previously been established to prevent an accidental clash in Syria during the fight against Islamic State.
The Pentagon noted that "extraordinary measures" were taken to prevent Russian casualties.
A Syrian state news agency said that the U.S. missile attack killed nine civilians, including four children, in areas near the base. The Syrian army reported extensive damage. In a statement, it described the attack as an act of "blatant aggression," saying it had made the United States "a partner" of Islamic State, the Nusra Front and other "terrorist organisations."
Iran on Friday denounced the strikes, in which two U.S. warships fired dozens of cruise missiles at an Assad-controlled airbase, as "destructive and dangerous," the Students News Agency ISNA quoted Foreign Ministry spokesman as saying.
"Iran ... condemns use of chemical weapons ... but at the same time believes it is dangerous, destructive and violation of international laws to use it as an excuse to take unilateral actions," ISNA quoted Bahram Qasemi as saying.
"Iran strongly condemns any such unilateral strikes... Such measures will strengthen terrorists in Syria ... and will complicate the situation in Syria and the region."