Story highlights Russia is a major backer of Syrian President Bashar al-Assad

Kremlin officials denounced the US strike, warned it may lead to repercussions

(CNN) Russian President Vladimir Putin has denounced the US strike against a Syrian government airbase as "aggression against a sovereign state in violation of the norms of international law."

The Russian Defense Ministry said Friday that it plans to bolster and increase the effectiveness of the air defense system in Syria following the attack.

"In order to protect the most sensitive objects of the Syrian infrastructure, a system of measures to bolster and increase the effectiveness of the Syrian armed forces' air defense systems will be implemented," ministry spokesman Igor Konashenkov said in a statement carried on state media.

Russia earlier announced it would suspend an October 2015 agreement aimed at minimizing risks of in-flight incidents between Russian and US-led-coalition aircraft.

The risk of a direct collision between the US and Russia in Syria had "significantly increased" since the US missile strike, Kremlin spokesman Dmitry Peskov said.

JUST WATCHED Meet the key players in Syria's civil war Replay More Videos ... MUST WATCH Meet the key players in Syria's civil war 02:22