The Syrian army is attacking hills overlooking a rebel-held Christian-majority village near the capital Damascus.
The British-based Syrian Observatory for Human Rights said fighters from the al Qaida-affiliated Jabhat al-Nusra or Nusra Front and the Qalamon Liberation Front still control Maaloula, an ancient village which is home to two of the oldest surviving monasteries in Syria.
Rami Abdul-Rahman, who heads the Observatory, said troops attacked the hills around Maaloula early today under the cover of heavy shelling.
Rebels captured the village on Saturday.
The battle has thrown a spotlight on the deep-seated fears that many of Syria's religious minorities harbour about the growing role of Islamic extremists on the rebel side in Syria's civil war.