SANAA, Yemen (AP) — Yemen’s Houthi rebels killed a tribal leader and four of his immediate family in the rebel-held capital, tribal leaders said on Saturday.

Armed rebels, led by Houthi leader Abu Khalid al-Raei, gunned down Sheikh Ali Abu Nashtan on Friday along with three of his sons and a sister outside their house in the Arhab district in northern Sanaa, they said.

The cause of killings wasn’t immediately clear, and the rebels declined to comment.

The Houthis have cordoned the area as tribesmen pledged revenge for the killing of Abu Nashtan family, said the tribal leaders who spoke on condition of anonymity for fear of reprisal.

The tribe of Arhab is a Houthi allay that helped the rebels’ takeover of Sanaa, triggering the Yemen civil war more than six years ago.

The tribe has fighters battling alongside the rebels in the war that pits the Iranian-backed Houthis against the internationally recognized government which is aided by a Saudi-led coalition.

Yemen plunged into chaos and civil war when the Houthi rebels took over Sanaa in 2014 from the internationally recognized government. A Saudi-led coalition allied with the government has been fighting the Houthis since March 2015.

The war in Yemen has spawned the world’s worst humanitarian crisis, leaving millions suffering from food and medical shortages. It has killed some 130,000 people, including fighters and civilians, according to a database project that tracks violence.