Syrian armed forces took territory east of Damascus on Monday, the Syrian Observatory for Human Rights said, on the third day of a fragile international attempt to halt nearly five years of fighting.
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A Syrian rebel spokesman said this was a violation of the truce deal in place.
The Observatory said that Syrian government forces took control of a strategically important piece of land between two neighbourhoods in the eastern Ghouta suburb of Damascus.
The capture of the land between Beit Nayim and Harasta al-Qantara came after Syrian and allied forces fought Islamist factions and the Al-Qaida-linked Nusra Front for around 24 hours, the Observatory said.
A fragile truce came into force in Syria early on Saturday, but the main opposition group has said that the deal could collapse because of continuing attacks by government forces.
Abu Ghiath al-Shami, spokesman for the Alwiyat Seif al-Sham group, part of a rebel alliance in the south, said government forces had been trying to storm the area in eastern Ghouta since the first day of the truce.
"This is a clear violation of the ceasefire," Shami said.
The cessation of hostilities, drawn up by Washington and Moscow, is a less formal arrangement than a ceasefire and is meant to allow peace talks to resume and aid to reach besieged communities.
The agreement does not include jihadist groups, such as Islamic State and the Nusra Front, and Russia --which is supporting Syrian forces with air attacks -- has made clear it intends to keep bombing them.
Eastern Ghouta is regularly targeted by the Syrian army and its allies. It is a stronghold of the Jaish al-Islam (Islam Army) rebel group, which is an influential member of the main opposition alliance, the High Negotiations Committee, and has been used as a launch pad for rocket and mortar attacks on Damascus.