At least 60 people, including 30 children and women, were killed in Russian and Syrian airstrikes on rebel-held areas in the central province of Homs, a monitoring group reported on Friday.
The Syrian Observatory for Human Rights said that the dead included 17 rebel fighters.
The overnight strikes were part of a major attack launched on Thursday by troops of President Bashar al-Assad, with Russian air support, against Islamist rebels on the northern outskirts of Homs.
Russia, an ally of al-Assad, has denied reports about civilian casualties resulting from its air campaign that started in Syria on September 30.
Moscow says its airstrikes are aimed at weakening the Islamic State extremist militia, which rules large parts of war-torn Syria.
Syrian Television reported that the ground attack by government troops was continuing Friday in the north and north west of Homs, mainly in the villages of Talibsseh and Khalidiyeh, which the broadcaster called "main hideouts of the terrorist groups"- a term referring to rebel fighters.
The broadcaster gave no further details.
The onslaught is aimed at securing a main highway linking Homs to neighbouring Hama, according to observers.
Rebels control an approximately 50-kilometer-wide swathe of territory north of Homs known as the "Rastan pocket."
Activists in central Syria posted online videos showing what they said were heavy air raids on areas in northern Homs.
A Talbisseh-based activist, who goes by the name of Hassan Abu Noah, said that more people were still buried under the rubble of buildings as a result of the airstrikes.
Rebels claimed they had killed the regime commander of the assault on Homs, Brigadier Maan Abu Deeb, during fighting in the area.
Their claim could not be independently verified.
Ousama Abu Zeid, a military adviser to the West-backed opposition Free Syrian Army, told dpa that the regime and their allies opened Friday a new battlefront on the southern edge of the northern province of Aleppo allegedly after they failed to advance in the northern areas of Homs.
More than 250,000 people are estimated to have been killed in Syria's conflict that started with peaceful pro-democracy protests in 2011.
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