Russian Defense Ministry confirmed the deaths of four Russian soldiers in clashes with extremists in eastern Syria in a statement on Sunday.

Russian and Syrian government troops and pro-government fighters have been battling members of the Islamic State terror group in the eastern Syrian province of Deir al-Zour, where the extremists resumed their attacks against government forces and their allies in recent weeks.

"Two Russian military advisers, who controlled fire of the Syrian battery, died at the scene. Five other Russian soldiers were wounded and immediately taken to a Russian military hospital.

"Doctors had been fighting for the lives of the two wounded servicemen, but they did not manage to save them," the Defense Ministry said, according to Russian state media Tass news agency.

The ministry said the battle occurred in the Deir al-Zour Governorate in the darkness when several terrorist groups attacked an artillery battery of the Syrian government forces.

The Syrian troops together with the Russian soldiers from the military advisers' group then entered the battle.

"During the fighting, which lasted for nearly an hour, the military killed 43 terrorists and six all-terrain vehicles with large-caliber armaments installed on them," the ministry said.

The statement did not say when the fighting occurred, although several reports suggested it may have taken place last Wednesday.

The Britain-based Syrian Observatory for Human Rights, a war monitoring group, said nine Russian soldiers died on Wednesday alongside at least 26 Syrian troops near Mayadeen town in Deir al-Zour, Agence France-Presse reported.

Media said Russia had provided military support to Syrian government forces, helping them to roll back mainstream rebels as well as the IS group.

The Russian Navy will maintain the composition of its grouping, permanently stationed in the Mediterranean Sea, at a balanced and efficient level, Russian Navy Commander Admiral Vladimir Korolev said on Sunday.

Russia's Sputnik news agency quoted the official as saying that the Russian grouping in the Mediterranean functions on the principle of regular rotation, while the number of ships in the grouping may vary depending on the tasks.

Earlier this month, Russian President Vladimir Putin said that the country's military vessels would be on permanent standby in the Mediterranean Sea due to a terrorist threat in Syria.

In the first three months of this year, the Russian Navy received a nuclear-powered ballistic missile submarine, three ships, two helicopters and 46 Kalibr cruise missiles.