Three Afghan aid workers employed by Catholic Relief Services were gunned down and killed in central Afghanistan, officials said on Tuesday.

The attack happened on Monday near the capital city of Ghor province, an area once relatively peaceful but now home to active Taliban and Islamic State militants.

The attack also wounded two employees, but the identity of the gunmen was unknown, said Iqbal Nezami, a spokesman for the provincial police.

Afghanistan, where the Taliban are seeking to restore Islamic rule after their 2001 ouster, is consistently considered one of the deadliest countries in the world for aid workers.

So far this year at least 12 have died in Afghanistan, compared to 15 in all of 2016, according to the United Nations.

"I call on all parties to ensure that those providing humanitarian assistance have safe access to the most vulnerable and can carry out their lifesaving work unhindered," UN humanitarian coordinator in Afghanistan Richard Peeperkorn said in a statement about the CRS attack.

CRS, which could not immediately confirm the report, provides humanitarian aid to more than 200,000 Afghans, with a focus on agriculture, education, and disaster response, according to its website.

Besides Ghor, CRS also operates in Afghanistan's Herat, Bamiyan, Daykundi, and Kabul provinces.