Fierce clashes have killed at least 21 rebel fighters and 26 regime troops in less than two days in the northwestern Syrian province of Idlib, a watchdog said on Thursday.
"Army troops and rebel fighters loyal to Al-Nusra Front, Ahrar al-Sham and the Dawud brigade are fighting fierce battles" at two checkpoints seized by rebels on Wednesday, said the Syrian Observatory for Human Rights.
"The fighting has killed at least 21 rebels, including 12 (non-Syrian) Arab fighters, among them an Al-Nusra Front commander," said the Britain-based monitoring group.
The violence also killed "at least 26 regime troops", it added.
Large swathes of Idlib province, near the border with Turkey, have slipped out of army control and into rebel hands.
Loyalists of Al-Nusra Front, which the United States lists as a "terrorist" organization, and which is not part of the mainstream rebel Free Syrian Army, often fight some of the toughest battles in the area.
Elsewhere in the country, clashes also erupted on the southern outskirts of the Syrian capital as army tanks pounded the area, monitors said.
"Regime tanks pounded the towns of Deir al-Asafeer, Daraya and Moadamiyet al-Sham," said the Observatory, which relies on a network of activists, doctors and lawyers for its reporting.
While clashes raged in the Palestinian refugee camp of Yarmuk and in Tadamun district early on Thursday, the army shelled the nearby Al-Hajar al-Aswad neighbourhood later in the day.
The army fired missiles at the rebel-held district of Khaldiyeh in Homs, more than six months into a suffocating siege of opposition districts in the central city, it said.
It also shelled the centre of Homs city and the district of Jobar on Thursday, while pounding the rebel-held nearby town of Rastan.
Thursday's violence came a day after at least 139 people were killed in violence across Syria, among them 51 civilians, 46 rebel fighters and 42 soldiers, according to the Observatory.
On Thursday, at least 37 people were killed in the country, according to a preliminary count.
The United Nations says a total of more than 60,000 people have died in the country's 22-month conflict.