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 on Sunday, as Syrian forces backed by regional allies and the Russian air force seized thousands of square kilometres from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group in the centre of the country.

Syrian government forces supported by Iranian-organised armed groups and the Russian air force have recaptured much of the country's central Homs province from ISIL in 2017.

Most of the province is desert. It contains several energy fields as well as phosphate minerals.

They are driving towards the city of Deir Az Zour, kept under siege by ISIL fighters since 2015.

OPINION: Can refugees return to Syria, as many want them to?

Shoigu, in an interview on Russian state-owned Rossiya 24 TV, said recapturing Deir Az Zour "will say a lot, if not everything, about the end of the battle with" ISIL.

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

Russia has provided air support for Syrian forces combating rebels and ISIL since 2015.

No let up in Damascus suburbs fighting

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

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

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

A Russian mediated ceasefire 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 neighbourhoods and towns have been destroyed. The opposition does not have an air force.

Also on Sunday, the al-Qaeda-linked Hay'et Tahrir al-Sham (HTS) said through its Ibaa' news agency that it had secured the release of 104 prisoners, among them 24 women, from government jails.

It said the release was negotiated as part of an agreement for the HTS to give up its positions in the Qalamoun Mountains, near Damascus, next month.

The Observatory said HTS released several of its own prisoners in exchange. They included soldiers and pro-government fighters.