Twelve members of Syria’s armed forces were killed in a suicide bomber attack attributed to a jihadist faction in a northwestern province, the Syrian Observatory for Human Rights announced Wednesday night.

The British-based NGO, which has a wide network of sources in Syria, said that “members of the regime forces, including an officer, were killed in a suicide bomber attack by Hayat Tahrir al-Sham (HTS) against regular army positions in the northern sector of province of Lattakia”.

This is the heaviest death toll in such action for the regime in a year, according to the Syrian Observatory for Human Rights.

HTS controls most of Idlib province, the last remaining anti-regime stronghold in the northwest of the country, and parts of the neighboring provinces of Aleppo, Hama and Lattakia.

More than five million people, most displaced from other provinces, live in areas outside Damascus’ control.

The HTS, designated a terrorist organization by Syria, the US and the EU, is often involved in clashes with regular army forces and their Russian allies.

It is the largest jihadist or rebel organization that remains active in northwestern Syria; however, several other organizations operate there, some of which are supported by Turkey.