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.

"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".

Elsewhere in the country, the Observatory said an aide to the labour minister was injured by an explosive device planted in his car in the Baramkeh district of Damascus.

The group, 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.