MOSCOW/DAMASCUS (Reuters) -Russian Foreign Minister Sergei Lavrov said on Tuesday that Moscow is deeply concerned by what he said was ethnic cleansing being carried out by "radical militant groups" in Syria.

The comments came as Syrian sources and Russian social media channels reported an apparent attack on one of the two military bases Moscow maintains in Syria.

"Radical militant groups are carrying out real ethnic cleansing, mass killings of people based on their nationality and religion," Lavrov said, according to a transcript of his comments published on the Foreign Ministry website.

There was no indication as to what groups he was referring to in his speech to a reception to mark the last day of the Easter period for Orthodox Christians.

Russia, which maintains two military bases in Syria, was a key backer of the government of ousted President Bashar al-Assad, who is now in Russia, where he was given asylum.

A March outbreak of sectarian violence in Syria's coastal northwest, which was an Assad stronghold, left hundreds of members of the former leader's Alawite sect dead.

Russia has attempted to maintain ties with Syria's new authorities, who have said that they may yet allow Moscow to retain use of the Hmeimim airbase and Tartus naval base.

Two sources in Syria familiar with the matter said that the Hmeimim airbase had come under attack earlier on Tuesday.

Military Informer, a pro-Russian Telegram channel that focuses on military matters, published what it said was video of a firefight between Russian troops and Syrian fighters at the base.

Reuters could not verify the footage.

The Syrian sources said that the motive for the attack on the airbase, which has sheltered Alawite refugees since the March attacks, was unclear. One, a Syrian security official based in the coastal city of Latakia, said an investigation was underway.

U.S. President Donald Trump this month ordered sanctions on Syria imposed during Assad's rule removed, in a major policy shift for Washington.

(Reporting by Timour Azhari and Suleiman al-Khalidi in Damascus, Writing by Felix Light; Editing by Sharon Singleton)