The foreign ministers of Iran and Russia voiced support for Syria on Saturday during a major attack by rebel groups, Iranian state media reported.

The Syrian army said on Saturday dozens of its soldiers had been killed in a major attack led by Islamist Hayat Tahrir al-Sham rebels who swept into the city of Aleppo, forcing the army to redeploy in the biggest challenge to President Bashar al-Assad in years.

Russia has promised Damascus additional military assistance that is expected to arrive in the next 72 hours, as rebel forces sweep the northwestern city of Aleppo, two sources in the Syrian military told Reuters.