MOSCOW (AP) — Russia's Ministry of Defense said Friday it has killed four Islamic State group leaders in an airstrike outside the eastern Syria city of Deir el-Zour.

Russia has been providing an air cover for Syrian President Bashar Assad's offensive on Islamic State group militants since 2015.

The defense ministry said its airstrike killed 40 militants, including four prominent warlords who gathered for a meeting of IS commanders in an underground bunker outside Deir el-Zour.

Heavy clashes are taking place between Syrian government forces and the Islamic State group around Deir el-Zour as militants fight back to reinstate a years-long siege of the city.

President Bashar Assad's troops on Tuesday broke the nearly three-year militant blockade of parts of the city, marking a significant advance against the extremists.

The Russian military named Abu Muhammad al-Shimali and Gulmurod Khalimov as two of the four IS leaders killed in the airstrike. The other two were not named in the statement.

Al-Shimali reportedly headed the movement of foreign fighters into Syria and processed the group's new recruits.

Khalimov, a colonel who received U.S. training while heading the riot police force in his native Tajikistan, has often been described as the IS' minister of war. The United States last year placed a $3 million bounty on his head.

The Russian defense ministry said the airstrike, which was carried out on an unspecified date, was ordered after Moscow received intelligence earlier this week about a meeting of senior IS commanders in the area.