Rebel forces have entered parts of Syria’s second-largest city, Aleppo, a monitoring group says, in the biggest offensive against the government in years.

The UK-based group says fighters set off two car bombs before advancing into neighbourhoods, without giving further details.

Government forces meanwhile say they have regained control of positions that had been taken by rebels in the Idlib and Aleppo countryside, Syria’s state news agency reported.

The Islamist militant group Hayat Tahrir al-Sham (HTS) and allied factions said they seized a number of towns and villages in Aleppo and Idlib provinces after launching the offensive on Wednesday.

The monitoring group Syrian Observatory for Human Rights (SOHR), which uses a network of sources on the ground in Syria, reported that Syrian and Russian planes carried out 23 air strikes on Aleppo city and the surrounding countryside on Friday.

The SOHR said 255 people, mostly combatants, had been killed in the fighting, the deadliest between rebels and pro-government forces in Syria for years.

It said opposition forces had taken control of more than 50 towns and villages since Wednesday.

Fighting which had raged since the civil war began in Syria in 2011 had largely wound down by 2020, when Turkey and Russia – Syria’s key ally – brokered a ceasefire to halt a push by the government to retake Idlib.

That led to an extended lull in violence, but sporadic clashes, air strikes and shelling continued.

Idlib is the last remaining opposition stronghold and is home to more than four million people, many of whom have been displaced during the conflict and are living in dire conditions.