Yemen police have arrested 21 suspects in the killing of a local office head of the United Nations World Food Program (WFP) in Taiz, according to Yemen’s Interior Ministry.

Moayad Hameidi was shot dead in Turbah in the Taiz governorate. Turbah is a town near the coast of the Red Sea.

The Interior Ministry said Saturday that Yemen police in Taiz made the nearly two dozen arrests over a 24-hour period.

Cindy McCain, the WFP’s executive director, has strongly condemned Hameidi’s killing, describing him as “a dedicated humanitarian.”

Hameidi had served the WFP for 18 years in Sudan, Syria and Iraq, according to McCain.

“Those responsible must be held accountable. Aid workers should never be a target,” McCain said in a post on Twitter on Friday.

The WFP’s representative and country director in Yemen, Richard Ragan, referred to Hameidi’s killing as “a profound tragedy for our organization and the humanitarian community.”

Hameidi, who worked as the local office head of WFP in Yemen, was a Jordanian national and “working in Turbah, in the country’s southwest. He came under fire on Friday afternoon, and the identity of the assailants is not yet known,” the UN said in a statement on Friday.

Hameidi had recently arrived in Yemen “to assume a new job as head of the agency’s office in Taiz,” according to the UN.

Years-long conflict divides Yemen

Taiz is the country’s third largest city and has been under siege by Iran-backed Houthi rebels for around seven years, “creating a blockade for essential goods and humanitarian supplies for the city’s residents,” the UN said.

Yemen’s conflict has been festering for more than a decade. In 2012, protesters unseated then-President Ali Abdullah Saleh a year after the 2011 Arab Spring uprisings swept through the region.