Updated

The United Nations peacekeeping chief says the U.N. mission in Congo will pursue the Allied Democratic Forces rebels who killed 15 peacekeepers there earlier this month.

Jean-Pierre Lacroix made the announcement Tuesday while visiting eastern Congo. He told peacekeepers that the U.N. will protect them and seek justice. He said peacekeepers will reinforce their presence and weaponry in Semuliki to neutralize rebel groups in the region.

The Dec. 7 attack was the deadliest single assault on a U.N. peacekeeping mission in nearly 25 years, killing 15 Tanzanian peacekeepers and at least five Congolese soldiers.

Uganda and Congo launched a joint military operation last week against the ADF rebels.

Human Rights Watch says the rebel group has killed more than 1,000 people in eastern Congo since October 2014.