The U.S. warned Russia last week that it was ready to take military action against Syria if President Bashar Assad uses chemical weapons to recapture Idlib, one of the last rebel-held areas in the war-torn country. The Russian Defense Ministry, which called the allegations of a chemical attack a false flag operation, said on Monday it had noticed Washington was building up its military forces in the Middle East in preparation for possible attacks.

The Russian Navy has dispatched the biggest task force to the Mediterranean Sea since the start of Russia's intervention in the Syrian conflict in anticipation of what Moscow says is a possible U.S. strike on regime forces.

To prepare for possible U.S. strikes, the Kommersant newspaper reported that the Russian military had moved its Tor-M2 surface-to-air missile systems and two frigates carrying Kalibr cruise missiles to Syria.

“This is the most powerful grouping since the beginning of the operation,” the pro-Kremlin Izvestia newspaper said.

A total of 10 surface ships and two submarines are on patrol in the Eastern Mediterranean off the Syrian coast, Izvestia cited the Defense Ministry as saying. One tanker and an unspecified number of other vessels are en route to the Mediterranean, the report said.

CNN quoted the Pentagon spokesman as saying that the Russian Defense Ministry report on a U.S. build-up is “nothing more than propaganda.”

“That does not mean, however, that we are unprepared to respond should the President direct such an action,” spokesman Eric Pahon was quoted as saying.

The U.S. has carried out two missile strikes against Syrian government forces since 2017 in retaliation to what it said were chemical attacks.

Russia launched an air and sea mission in support of the Syrian government in mid-2015.

Reuters contributed reporting.