Russia's Defence Ministry said its air force had carried out strikes on Syrian rebels in support of the country's army, Russian news agencies reported on Saturday, amid an advance by forces opposed to Syrian President Bashar al-Assad.

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.

The surprise attack, led by the Islamist group Hayat Tahrir al-Sham, was the boldest rebel assault for years in the civil war that has been frozen.