Islamic State militants have demolished a monastery in the central Syrian province of Homs, a monitor said on Thursday, in a strategically located town the jihadist group wrested from government control earlier this month.

The group has also transferred several dozen Christians, captured during its offensive, to a location near its stronghold in northeastern Syria.

Militants used bulldozers to raze the monastery in the town of Qaryatain, which they had captured in early August, the Britain-based Syrian Observatory for Human Rights said.

Government warplanes were still pounding the area with air strikes two weeks after Islamic State took the town, the monitor said.

Qaryatain is near a road linking the ancient Roman city of Palmyra to the Qalamoun mountains, along the border with Lebanon.

The hardline militant group has been gaining ground in the desert areas east and south of Homs after it took over Palmyra last May.

The Syrian army has launched a large-scale counter-offensive to recapture the city, which lies in a region where some of Syria's largest gas fields are located, but so far it has made no significant advances.

Islamic State militants captured 230 people including dozens of Christian families after taking Qaryatain, the monitor reported at the time.

Of those captured, 48 had been released and 110 were transferred to Raqqa province, whose capital city Raqqa is the militants' Syria stronghold, the monitor said on Thursday.

The Christians would be given the choice of conversion to Islam or paying "jizya", a tax on non-Muslims, the monitor said, citing "informed sources."

The fate of the remaining 70 people captured after the seizure of Qaryatain was unclear.

Among them were 45 women and 19 children, including 11 families, some of whom were on a militants' wanted list, said the monitor, which tracks the violence of Syria's civil war through an extensive network of sources on the ground.

Islamic State has killed members of religious minorities and Sunni Muslims who do not swear allegiance to its self-declared "caliphate". They also consider Christians as infidels.