Syrian warplanes bombed the central city of Homs on Thursday, with insurgents and troops battling on the ground as regime forces pressed an assault on rebel-held neighbourhoods, an NGO said.
The Syrian government meanwhile said it had asked the International Committee of the Red Cross to cooperate to help evacuate civilians in the city.
"Warplanes carried out two raids against the Khaldiyeh neighbourhood of Homs, and both Khaldiyeh and the Old City were under heavy rocket fire producing the sound of explosions and plumes of smoke," the Syrian Observatory for Human Right said.
"Sporadic clashes were ongoing between rebels and regime forces on the outskirts of Khaldiyeh," the watchdog added.
Regime forces began a campaign to retake several rebel-held neighbourhoods of Homs, often dubbed the capital of the uprising against President Bashar al-Assad, on Saturday.
The neighbourhoods being targeted have been under siege by regime troops for more than a year, and many civilians have fled, but concerns have been raised about those who remain.
On Tuesday, UN Secretary General Ban Ki-moon expressed fears for 2,500 civilians "trapped" in the city, which is home to a patchwork of religious communities.
Ban called on "the warring sides to do their utmost to avoid civilian casualties and to allow immediate humanitarian access, as well as opportunities for trapped civilians to leave without fear of persecution".
Syria's foreign ministry said it "called on the Red Cross in cooperation with the Red Crescent to work to evacuate civilians trapped in the Old City of Homs," a source told state media.
The foreign ministry source told state TV that civilians in the city were being "used as human shields by armed terrorist groups".
Elsewhere in the country, the Observatory said the assistant labour minister Rakan Ibrahim was injured by an explosive device planted in his car in the Baramkeh district of Damascus.
The labour ministry confirmed the incident involving Ibrahim, saying he had been admitted to hospital and was undergoing operations.
The Observatory, which relies on a network of activists, doctors and lawyers on the ground, also reported shelling on the Palestinian Yarmuk refugee camp in the capital.
In southern Daraa province, the group said six people were killed in shelling on the town of Sheikh Miskeen.
The group said at least 110 people were killed throughout Syria on Wednesday, including 35 civilians, 52 rebels and 23 regime forces.