Seventeen aid workers were killed by Somalia’s Al Shabaab last year, the United Nations said Tuesday night.

The number of attacks on aid agency staff there rose to 140 last year from 75 in 2014, when 10 workers were killed. Somalia has been mired in conflict since civil war broke out in 1991 and is one of the most difficult countries for relief agencies to operate in.

The Islamist militant group al Shabaab has waged a decade-long insurgency against the Somali government, which is backed by African Union troops. Warring parties have deliberately targeted aid workers and manipulated aid for political gain.

“Attacks and threats against humanitarians increased,” the U.N. Office for the Coordination of Humanitarian Affairs said in a bulletin, adding that there were also 18 injuries, 11 abductions and 38 arrests in 2015 involving aid agency staff.

Fighting, poor infrastructure and funding shortages make it difficult to reach the 40 percent of Somalia’s 12 million people needing aid, OCHA said.

Some parts of south-central Somalia are only accessible by air, driving up the cost of delivering essentials like food.

African Union troops have taken major towns from al Shabaab but the group still controls swathes of countryside and has laid siege to urban areas.