Russia’s navy dispatched a dozen vessels to the Eastern Mediterranean after the U.S. national security adviser reportedly warned that Washington was ready to attack Syria if chemical weapons were used in the country. Russian media called the reinforcement “the most powerful grouping” dispatched to Syria since Moscow entered the conflict in September 2015.

The NATO military alliance has called for restraint amid reports that Russia has amassed the biggest naval task force near Syria since the start of Moscow's intervention in the country’s civil war.

NATO spokeswoman Oana Lungescu confirmed the Russian buildup in the Mediterranean to Israel’s Haaretz newspaper but declined to “speculate on the intention of the Russian fleet.”

“[I]t is important that all actors in the region exercise restraint and refrain from worsening an already disastrous humanitarian situation in Syria,” she was quoted as saying.

Reuters said its analysis of shipping traffic showed at least three frigates, a missile corvette and a landing ship being deployed to the Mediterranean this month.

The reinforcement, which includes warships carrying Kalibr cruise missiles, comes amid reports that Syrian President Bashar Assad is preparing an assault on Idlib, one of the last rebel-held areas in the country.