The Russian Foreign Ministry said following Donald Trump's decision to fire 59 cruise missiles at a military target in Syria on Thursday, Moscow was suspending a memorandum with the US that prevented incidents and ensured flight safety.
Under the memorandum, signed after Russia launched an air campaign in Syria in September 2015, Russia and the US had exchanged information about their flights to avoid incidents in the crowded skies over Syria — where Russia has several dozen warplanes and batteries of air-defence missiles.
The so-called "deconfliction line" between the US and Russia has been credited by a number of US pilots with helping keep them safe. While the US air campaign in Syria tends to steer clear of areas where Russia is in operation, ending such co-operation is seen by activists in the region as a worrying turn.
The US-side of the information sharing exercise is based in the al-Udeid Air Base in Qatar, which is also the forward base from US Central Command, which takes the lead in co-ordinating coalition strikes. The US maintains radar coverage and other means of scanning the skies, but without being able to share coordinates and other data with the Russians, the chances of a confrontation increases.
Russian President Vladimir Putin, a staunch ally of Syrian President Bashar al-Assad, condemned the US strikes near Homs, saying they broke international law and have seriously damaged US-Russia relations.
A statement from the Russian Foreign Ministry read: "The very presence of US troops and other countries on the territory of Syria, without the consent of the Government or the decision of the UN Security Council is a gross, obvious and unwarranted violation of international law.