A drone strike believed to have been carried out by the US in northwestern Syria killed a high-profile Tunisian extremist, a monitor said on Tuesday. The attack on Monday in the rebel-held province of Idlib killed Sayyaf Al Tunsi, according to the Syrian Observatory for Human Rights. "The drone targeted the vehicle of the Hurras Al Deen leader in Idlib, killing him," said Observatory chief Rami Abdel Rahman. Hurras Al Deen is a small but powerful armed group led by Al Qaeda loyalists. It is a rival of Hayat Tahrir Al Sham, an alliance of extremist factions dominated by former members of Jabhat Al Nursa, once Al Qaeda's Syria affiliate. The two groups co-operate from time to time. Al Tunsi was a former Nusra leader who was sidelined for his alleged responsibility for the 2015 massacre of 20 Druze villagers in Idlib province. There was no confirmation of the strike from the US, nor the coalition it leads in the region to fight ISIS. Mr Abdel Rahman said Al Tunsi's car was targeted by a Ninja R9X missile, a modified Hellfire on which the explosive head is replaced with blades to minimise collateral damage. Use of the R9X is consistent with US air strikes against senior militants in northern Syria.