Secretary General Antonio Guterres urged restraint as Russia accused the US of encouraging "terrorists" in the region with its unilateral actions.
Russia has promised to strengthen Syria's anti-aircraft defences.
In the first direct US military action against Syria's government, at least six people were reported to have been killed after 59 Tomahawk cruise missiles were fired at Shayrat air base in Homs about 4.40am local time from destroyers in the eastern Mediterranean.
