REUTERS - Russia’s Ministry of Defense said on Thursday it would hold a major naval exercise in the Mediterranean Sea from Sept. 1 to Sept. 8, the TASS news agency reported, a move that coincides with rising tensions between Moscow and Washington over Syria.

Russia has this month been actively expanding its naval forces in the Mediterranean, part of what a Russian newspaper on Tuesday called Moscow’s largest naval build-up since it entered the Syrian conflict in 2015.

TASS cited the defense ministry as saying that 25 warships and submarines and 30 planes, including fighter jets and strategic bombers, would take part, and that the drills would involve anti-aircraft, anti-submarine and anti-mining exercises.

The drills come amid rising tensions between Russia and the West over Syria’s northern rebel-held region of Idlib. A source said on Wednesday that Russia’s ally, Syrian President Bashar al-Assad, was preparing a phased offensive there.

Russian Foreign Minister Sergei Lavrov said on Wednesday that militants in Idlib had to be liquidated, describing them as “a festering abscess”.

“In the interest of ensuring the security of shipping and aircraft flights in line with international law, the areas of the exercise will be declared dangerous for shipping and flights,” TASS quoted the defense ministry as saying.

Anatoly Antonov, Russia’s Ambassador to the United States, said earlier on Thursday that he had told U.S. officials that Moscow was concerned by signs that the United States was preparing new strikes on Syria.