A vigilante group linked to al Qaeda in the eastern port city of Mukalla has decreed a ban on trading the mild narcotic leaf qat chewed as a cherished Yemeni pastime, in a sign of the group’s growing territorial hold amid the country’s civil war.

A security vacuum in Yemen’s east is giving al Qaeda freer rein to enforce social rules and make arrests, residents say.

A newly-formed body of armed tribesmen and Sunni Muslim clerics assumed control over much of eastern Yemen’s oil-producing province of Hadramawt after army units abandoned it last month. […]