The Russian Navy's frigate Admiral Grigorovich sails in the Bosphorus, on its way to the Mediterranean Sea, in Istanbul, Turkey August 25, 2018. REUTERS/Yoruk Isik

MOSCOW (Reuters) - Russia has deployed several frigates to the Mediterranean via the Bosphorus, an analysis of shipping traffic showed, part of what a Russian newspaper on Tuesday called Moscow’s largest naval buildup since it entered the Syrian conflict in 2015.

The reinforcement comes as Russia’s ally, Syrian President Bashar al-Assad, is believed to be considering an assault on the last big rebel-held enclave, Idlib in the north.

Russia has accused the United States of building up its own forces in the Middle East in preparation for a possible strike on Syrian government forces.

On Saturday, the Admiral Grigorovich and Admiral Essen frigates sailed through Istanbul’s Bosphorus towards the Mediterranean, Reuters pictures showed.

The day before, the Pytlivy frigate and landing ship Nikolai Filchenkov were pictured sailing through the Turkish straits that connect the Black Sea with the Mediterranean. The Vishny Volochek missile corvette passed through earlier this month.

The Izvestia newspaper said Russia had gathered its largest naval presence in the Mediterranean Sea since it intervened in Syria in 2015 turning the tide in Assad’s favor.

The force included 10 vessels, most of them armed with long-range Kalibr cruise missiles, Izvestia wrote, adding that more were on the way, and that two submarines had also been deployed.