Five civilians were killed in Russian air strikes backing Syrian regime forces as they chipped away at the country’s last major rebel bastion in fighting that cost dozens of lives Monday, a war monitor said.

The death last month of the leader of al-Qaeda’s Yemen branch in a U.S. drone strike has dealt a blow to the group’s ambitions, but Western officials and analysts warn that a recent escalation in Yemen’s conflict could allow it to regroup to some extent.

When the grip of Iraq’s Tehran-backed Shi’ite Muslim parties and militias threatened to slip following the killing of Iranian general Qassem Soleimani, they turned to an unpredictable rival.