Russia has announced it will hold a week-long military drill in the Mediterranean off the coast of Syria, as the UN warned a “perfect storm” was brewing in the battle for the country’s last rebel stronghold.

Moscow’s defence ministry said it will deploy 25 ships, including a Marshall Ustinov missile cruiser, and 30 jets for the exercises, which would focus on anti-air and anti-submarine defence.

The armada is the biggest task force sent by President Vladimir Putin’s government since Russia’s intervention in the Syrian conflict in 2015.

Kremlin spokesman Dmitry Peskov said the drills were justified by the situation around the Idlib, the northwestern province where Syrian and allied Russian forces are expected to soon launch an offensive.

"The situation in Syria has the significant potential of exacerbating,” Mr Peskov said. “Nothing good can come out of this terrorist hotbed that formed there, if no one acts. So tightened security measures are totally justified and reasonable.”