Story highlights Clashes are underway in government-held areas of Aleppo

At least 21 people were killed during intense fighting since Friday, rights group says

(CNN) At least 21 people have been killed since Friday in tit-for-tat clashes between Syrian regime forces and rebels in the besieged city of Aleppo, the UK-based monitoring group Syrian Observatory for Human Rights (SOHR) reported.

The rights group said army forces, backed by Hezbollah militants and seeking to put down a new rebel offensive , on Saturday retook pockets of neighborhoods where rebel fighters a day earlier unleashed a series of deadly car bombings and mortar assaults.

The rebel offensive was repelled in part by renewed Russian and Syrian airstrikes targeting parts of west, south and southwest Aleppo, SOHR reported.

Opposition social media on Friday showed video of rebel tanks, armored personnel carriers, Grad rockets and artillery firing from the southwest of the city.

On Saturday, airstrikes and shelling in the Salaheddine neighborhood killed at least three people and wounded five others, according to SOHR. Rebels countered with intense shelling that wounded more than 12 people in regime-held positions in western parts of the city.

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