Two Red Cross employees were shot dead in the northern Yemeni province of Amran by an unknown attacker on yesterday, the international aid group said, in a rare case of violence against humanitarian workers in a five-month war.

The pair were Yemeni nationals and were returning from aid work in the far northern province of Saada, spokesman Adnan Hizam said by telephone.

“Two of our colleagues were shot and killed in their car ...as they were travelling back from Saada to Sanaa,” Hizam said from the Yemeni capital.

A statement from the group identified the murdered employees as a field officer and a driver, adding that they believed a lone gunman was responsible but the reason for the attack remained unclear.

A civil war in Yemen escalated in March when a coalition of Arab states led by Saudi Arabia intervened to prevent the Houthis from expanding and to reinstate the country's government from its exile in Riyadh.

“The ICRC condemns in the strongest possible terms what appears to have been the deliberate targeting of our staff," said the head of the ICRC delegation in Yemen, Antoine Grand.