(CNN) — At least 12 people — five United Nations peacekeepers, two U.N. staff members and five civilian contractors — were killed Tuesday in an attack in South Sudan, U.N. officials said.

The five U.N. peacekeepers were from India and were part of a convoy of 32 Indian peacekeepers that came under attack in an ambush in Jonglei, South Sudan, according to Syed Akbaruddin, spokesman for India’s Ministry of External Affairs. Four others in the Indian group were injured, he added.

The U.N. said a total of at least nine peacekeepers and civilians were injured “and some remain unaccounted for.”

The U.N. mission in South Sudan said the attack was “an ambush by unidentified assailants” near the settlement of Gumuruk in Jonglei, according to a U.N. news release. The area was frequently patrolled “to provide protection to civilians, as well as providing armed escorts to humanitarian aid convoys,” the release said.

UN Secretary-General Ban Ki-moon “is appalled by the attack,” the U.N. said, adding that Ban stressed “the killing of peacekeepers is a war crime that falls under the jurisdiction of the International Criminal Court.”

India is making arrangements to have the bodies of those killed returned to the country, according to Akbaruddin.

There are 2,200 Indian peacekeepers deployed in South Sudan, he said.

CNN’s Sumnima Udas contributed to this report.