Syrian troops and allied militiamen pressed on with an offensive against Islamic State militants in central Syria on Monday, clashing with the extremists around the town of Qaryatain a day after it was captured by pro-government forces.

The push into Qaryatain took place under the cover of Russian airstrikes and dealt another setback to ISIS in Syria a week after the army retook the historic town of Palmyra from the group. Syria's state news agency, SANA, said the army was fighting ISIS militants in areas around Qaryatain Monday, as well as in farms east and north of Palmyra.

The capture of Qaryatain deprives ISIS of a main base in central Syria and could be used by government forces in the future to launch attacks on ISIS-held areas near the Iraqi border.

Qaryatain used to be home to a sizable Christian population and lies midway between Palmyra and the capital, Damascus. Activists said last summer that Qaryatain had a mixed population of around 40,000 Sunni Muslims and Christians, as well as thousands of internally displaced people who had fled from the nearby city of Homs. Many of the Christians fled the town after it came under attack by ISIS.

Dozens of Qaryatain's Christians and other residents have been abducted by ISIS. While the town was under Islamic State control, some were released while others were made to sign pledges to pay a tax imposed on non-Muslims.

Also on Monday, a senior U.S. official said it was a U.S. airstrike that killed a senior Al Qaeda official Sunday night. The strike killed at least 21 other militants in Idlib province, a jihadist stronghold in northern Syria.

The official spoke on condition of anonymity because he wasn't authorized to discuss the operation.

The SITE Intelligence Group, which monitors jihadi websites, said Abu Firas al-Souri died in U.S. strikes while the Britain-based Syrian Observatory for Human Rights said the jets were thought to belong to the Syrian or Russian Air Forces. It said they targeted the headquarters of Jund al-Aqsa, an extremist group that fights alongside All Qaeda's Syrian affiliate, the Nusra Front.

Al-Souri was the former official spokesman for the Nusra Front, the group reported on social media Monday.

A 2014 biographical video about al-Souri, obtained by SITE, says he used to represent Osama bin Laden in Pakistan after he met the Al Qaeda founder in Afghanistan during the jihad against the Soviet Union in the 1980s.

Al-Souri, born outside Damascus in 1949, followed the path of many Syrian jihadists. A graduate of the country's military college, he trained jihadist cells in the country between 1977 and 1980, heading several operations against the authorities for the latter part of that period. He was expelled from the Syrian military, in part because of his Islamist ties, in 1979.

He fled to Jordan in 1980 then to Afghanistan in 1981 where he trained jihadists coming to the war-torn country from across Asia and the Arab world. He became an associate of bin Laden and Abu Musab al-Zarqawi, a senior Al Qaeda commander who led the organization's affiliate in Iraq following the 2003 U.S. invasion.

Al-Souri participated in a number of major military operations in Afghanistan and Pakistan before transferring to Yemen in 2003. In 2013, the Al Qaeda leadership transferred him to Syria to mend the growing rift between the group and the Islamic State.

A media outlet belonging to the Lebanese militia Hezbollah said al-Souri's son was also killed in the air strikes.

Hezbollah has sent thousands of its fighters to fight alongside Syrian government forces in the country's five-year civil war. The group was reported to have lost a dozen soldiers in fierce fighting in northern Syria last weekend as jihadist groups alongside rebel militias mounted an offensive against several government positions.