BEIRUT, Lebanon — The Syrian government has increased the size of the territory under its control by two and a half times in just two months, Russian Defense Minister Sergei Shoigu said Sunday, as Syrian forces backed by regional allies and the Russian air force seized thousands of square kilometers from the Islamic State group in the center of the country.

Syrian government forces supported by Iranian-organized militias and the Russian air force have recaptured much of the country’s central Homs province from the Islamic State group in 2017. Most of the province is desert. It contains several energy fields as well as phosphate minerals.

They are driving toward the city of Deir el-Zour, kept under siege by IS militants since 2015.

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Shoigu, in an interview on Russian state-owned Rossiya 24 TV, said recapturing Deir el-Zour “will say a lot, if not everything, about the end of the battle with” the Islamic State group.

The Britain-based Syrian Observatory for Human Rights monitoring group reported on Sunday that government forces killed at least 25 IS militants in a commando operation in the desert region. It said the soldiers rappelled down from helicopters to ambush the militants, under the cover of Russian air strikes.

Russia has provided air support for Syrian forces combating rebels and the Islamic State group since 2015.

Elsewhere, a rebel faction said it killed 20 army soldiers outside the Syrian capital in a tunnel blast as the battle for Damascus’s northeastern suburbs showed no signs of letting up.

Wael Olwan, spokesman for the Failaq al-Rahman faction, said the operation took place before dawn Sunday.

The Britain-based Syrian Observatory for Human Rights monitoring group said 16 soldiers were reported missing because of the blast.

A Russian mediated cease-fire announced last month has failed to quell the fighting between the government and rebels on the northeastern edges of Damascus. The Failaq al-Rahman opposition faction says it is not party to the agreement, and the government says it is fighting terrorists.

The government has leaned on its air force and its ground-to-ground missile systems to push rebels out and away from the capital. Several neighborhoods and towns have been destroyed. The opposition does not have an air force.