Two Tanzanian peacekeepers with the special UN intervention force in the Democratic Republic of Congo have been killed in an ambush, the UN has said.
Thirteen others were wounded in the attack by suspected Ugandan Allied Democratic Forces Islamist rebels.
It was second attack on UN forces in the eastern North Kivu region in 48 hours after a helicopter carrying a senior commander came under fire.
The UN has promised a "robust" military offensive in response.
Earlier, the Congolese army said it had killed 16 Allied Democratic Forces (ADF) rebels during fierce fighting over the weekend.
The ADF was formed in late 1990s by a puritanical Muslim sect in the Ruwenzori mountains of western Uganda.
The army launched a UN-backed offensive against the group - one of many based in the mineral-rich east of DR Congo - after they were accused of hacking more than 200 villagers to death near Beni in a series of massacres late last year.
The UN intervention brigade convoy came under attack on Tuesday about 12km (seven miles) from Beni, UN peacekeeping spokesman Nick Birnback told the BBC's Newsday programme.
Four peacekeepers have also been reported missing following the ambush.
Martin Kobler, head of the UN mission in DR Congo (Monusco), said in a tweet in French. "I won't tolerate any more of these repeated attacks against blue helmets in Beni territory.
"Monusco will carry out robust offensive operations," he said.
UN peacekeeping in DR Congo:
- 2000 First deployed: Strength - more than 5,500
- 2013 Extra UN intervention bridge deployed: Strength - 3,000
- 2015 Current mission, with main aim to protect civilians: 20,878, including brigade
In a statement, the spokesman for UN chief Ban Ki-moon condemned the killings and the "continuing atrocities perpetrated by the ADF against defenceless civilians in the Beni area".
The 3,000-strong UN intervention brigade, which is part of Monusco, is mandated to mount offensive operations against rebel groups in DR Congo.
It deployed in July 2013 in the wake of the uprising of the M23 rebel group which had forced some 800,000 people from their homes.
Together with the Congolese army it managed to defeat the M23 fighters later that year - and has since been targeting other groups in the east.