Russians warships in the Mediterranean Sea have fired four cruise missiles at the Islamic State group’s (I.S.) positions in Syria, the Russian defense ministry said on Wednesday.

The announcement came as Syrian government troops pushed ahead in their offensive against I.S. and militants in central and northern Syria.

Moscow said in a statement that the Admiral Essen frigate and the Krasnodar submarine launched the missiles at I.S. targets in the area of the ancient town of Palmyra. There was no information on when the missiles were launched.

Syrian troops have been on the offensive for weeks in northern, central and southern part of the country against I.S. and U.S.-backed rebels under the cover of Russian airstrikes, gaining an area almost half the size of neighboring Lebanon.

Most recently, Syrian troops and their allies have been marching toward the I.S. stronghold of Sukhna, about 60 kilometers (37 miles) northeast of Palmyra.

The strategic juncture in the Syrian desert aids government plans to go after I.S. in Deir el-Zour, one of the militants’ last major strongholds in Syria. The oil-rich province straddles the border with Iraq and is the extremist group’s last gateway to the outside world.

Russia, a staunch Damascus ally, has been providing air cover to Syrian President Bashar Assad’s offensive on I.S. and other insurgents since 2015. Moscow had fired cruise missiles from warships in the past, as well as from mainland Russia against Assad’s opponents.

As the fighting against I.S. militants is underway near Palmyra, Syrian troops clashed with U.S.-backed rebels in the country’s south on Wednesday, according to the Britain-based Syrian Observatory for Human Rights and Mozahem al-Salloum, of the activist-run Hammurabi Justice News network that tracks developments in eastern Syria.

The fighting came days after the United States told Syrian government forces and their allies to move away from an area near the Jordanian border where the coalition is training allied rebels.

The warning comes less than two weeks after the Americans bombed Iranian-backed troops there after they failed to heed similar warnings.

Navy Capt. Jeff Davis, a Pentagon spokesman, said on Tuesday that the U.S. dropped leaflets over the weekend telling the forces to leave the established protected zone.

Syrian troops backed by Russian airstrikes captured Palmyra in March last year and Moscow even flew in one of its best classical musicians to play a triumphant concert at Palmyra’s ancient theater. I.S. forces, however, recaptured Palmyra eight months later, before Syrian government troops drove them out again in March this year.

Russia’s defense ministry said its Wednesday statement that the strikes successfully hit I.S. heavy weapons and fighters whom the group who had deployed and moved to Palmyra from the I.S. stronghold of Raqqa, the de facto capital of the Sunni militant group and its self-proclaimed caliphate.

Moscow said it had notified the U.S., Turkish and Israeli militaries beforehand of the upcoming strike. It added that the Russian strike was promptly executed following the order, a testimony to the navy’s high readiness and capabilities.

Russia has been busy mediating between Assad and Turkey and the West who seek his removal. Earlier this month Russia, Iran and Turkey agreed to establish safe zones in Syria, signing on to a Russian plan under which Assad’s air force would halt flights over designated areas across the war-torn country. Russia says maps delineating the zones should be ready by June 4.