WASHINGTON

The U.S. strongly condemned Monday attacks against humanitarian volunteers in Yemen and Syria.

"We remind all armed groups of their obligations under international humanitarian law to respect and protect civilian personnel engaged in humanitarian efforts and we call on all parties to ensure their unimpeded access to people in need of assistance," State Department spokeswoman Marie Harf said in a press release.

Two brothers, Khaled Ahmed Bahuzaim and Mohammed Ahmed Bahuzaim, working for the Yemen Red Crescent Society were shot dead April 3 in Aden, Yemen, while assisting wounded victims, according to the International Federation of Red Cross and Red Crescent Societies, or IFRC.

The IFRC said that on the same day, two Syrian Arab Red Crescent volunteers, Ibrahim Eid and Mohamad Ahmad Kamouaa, were also killed while on duty in Idlib, Syria. 

"These aid workers were retrieving bodies of the deceased and preparing shelters for people fleeing the fighting in the city, while in Yemen they were killed by sniper fire while attempting to evacuate the wounded," Harf said.

Attacks by armed groups and the Syrian regime against innocent civilians and aid organizations is "unacceptable and must not be permitted by the international community," she said. "Impartial and neutral humanitarian organizations must be allowed to continue their critical work saving lives free from threats from armed groups."

According to Aid Worker Security Report 2014, 2013 set a record for violence against humanitarian volunteers with 251 separate attacks that affected 460 aid workers -- 155 were killed, 171 seriously wounded, and 134 were kidnapped. 

There were 66 percent more attacks against humanitarian aid workers in 2013 compared to 2012. 

At least 79 aid workers were killed in the first eight months of 2014, reflecting figures available through August.