Syrian rebels have reportedly breached the city of Aleppo itself, according to open source analysts and the Syrian Observatory for Human Rights, as thousands of insurgent fighters make startling advances in their surprise offensive against President Bashar al-Assad’s regime in northwestern Syria.

Videos circulating online appear to show rebels in the outskirts of Aleppo, Syria’s second largest city. The rebels launched a major offensive three days ago and since then have moved swiftly towards Aleppo, capturing swathes of countryside and villages.

The offensive reignites the frontline in Syria for the first time in years and the Assad regime and its allies Russia and Hezbollah appear to be struggling to hold it back.

The Syrian Observatory for Human Rights has reported heavy fighting between regime forces and the rebels, led by the jihadist group Hayat Tahrir al-Sham (HTS), which controls the rebel holdout of the neighboring Idlib province.

Although Syria’s civil war never ended, the frontlines have been largely frozen for years, while Russia and the regime continued to bomb rebel areas.

This is a developing story. Check back for updates.

-ABC News’ Patrick Reevell and Victoria Beaule contributed to this report.