* Soldiers are from Chad, U.N. source says
* Six peacekeepers "very seriously wounded"
* Blast follows a week of attacks against U.N. in area (Adds U.N. statement)
By Souleymane Ag Anara
BAMAKO, Sept 2 (Reuters) - Four United Nations soldiers were killed on Tuesday and a further 15 were wounded when the convoy they were traveling in struck a landmine in an area of northern Mali where attacks are on the rise, a spokesman for the country's U.N. mission said.
The contingent had left a U.N. base in the town of Aguelhok and were traveling south when the blast occurred at around noon local time (1200 GMT), about 30 km (19 miles) north of Kidal, the spokesman said.
He had earlier said the convoy was traveling north from Kidal when it struck the mine.
"The toll is four dead and 15 wounded, including six very seriously," Olivier Salgado told Reuters. "This is only a provisional toll because we do not know what will happen with six of the very seriously wounded."
Salgado said the mission, known as MINUSMA, was investigating the incident but could not immediately say who was responsible for the attack.
The soldiers were part of a contingent from Chad, the United Nations press office said.
"The Secretary-General condemns this incident which is the latest in a series of attacks against U.N. personnel and contractors in the Kidal Region in the past week," the U.N. statement said.
"These attacks will not alter the determination of the United Nations to support the Malian people in their search for peace," it added.
U.N. peacekeepers continue to be targeted in sporadic attacks despite a successful French-led intervention last year that drove back Islamists who had taken advantage of a separatist uprising to seize Mali's desert north.
Tuesday's incident occurred amid a rise in violence against U.N. personnel and vehicles in and around Aguelhok. Thirteen rockets have been fired at the town's U.N. base alone over the past week though those attacks had not resulted in any deaths or injuries, Salgado said.
"I am outraged by such a surge in violence against peace soldiers," MINUSMA head Albert Koenders said in a statement. "A peace process is under way, some people want to compromise it. This is totally unacceptable and irresponsible."
Al-Qaeda linked Islamist fighters swept in and took control of the northern two-thirds of Mali in early 2012 following a military coup in the capital Bamako, which was launched in reaction to an uprising by Tuareg separatists.
The Tuareg groups, who initially worked with the Islamists, were later sidelined by their better armed allies. They were not targeted by last year's French-led military operation, and the government in Bamako agreed to open talks with them.
Negotiations between government representatives and the Tuareg groups resumed in Algeria's capital Algiers this week.
Malian Foreign Minister Abdoulaye Diop said on Monday that the government hoped to clinch a preliminary peace deal with the separatists within eight weeks.
Mali's northern desert region - called Azawad by the Tuareg rebels - has risen up four times in the last 50 years, with various groups fighting for independence or self-rule. (Reporting by Souleymane Ag Anara; Additional reporting by Joe Bavier in Abidjan and Louis Charbonneau in New York; writing by Joe Bavier; Editing by Daniel Flynn, Angus MacSwan and Bernard Orr)