Four UN peacekeepers have been killed and 15 injured by a landmine in northern Mali.
A spokesman for the UN's mission in Mali, Minusma, told the BBC that six of the wounded were in an "extremely serious condition".
He added that UN forces had faced "increased attacks" this week.
The UN mission took over responsibility for security in northern Mali last year after French troops forced Islamists from key towns.
Minusma said that the landmine detonated on Tuesday between 11:00 and 12:00 local time (11:00-12:00 GMT), about 30km (19 miles) from the town of Kidal, on the road to Aguelhok.
The latest attack is one of several on UN forces in recent weeks.
On Monday the UN's base in Aguelhok came under mortar fire and in a separate attack, four civilian contractors for the UN were injured when their vehicle drove over a landmine.
No-one has claimed responsibility for the most recent attacks on the UN, reports the BBC's Alex Duval Smith in Bamako.
But on Sunday, al-Qaeda in the Islamic Maghreb said it had been behind a series of attacks in July and August in the Timbuktu region, our correspondent adds.
France intervened militarily in Mali last year in a bid to drive out Islamists who had taken advantage of an uprising by Tuareg rebels in the north of the country.
The latest attack comes as peace talks resume in Algeria between Tuareg rebels and Mali's government.
However, the talks do not include groups demanding Islamic rule.