The blast caused the collapse of a bank guarded by the soldiers and set a U.N. armored vehicle on fire, the BBC reported.
Officials fear more casualties may found in the rubble of the bank.
The dead peacekeepers were part of the U.N. peacekeeping force known as MINUSMA, Voice of America said.
Kidal, in Mali's northeastern region, has been a stronghold for separatist Tuareg rebels but no group immediately claimed responsibility for the attack.
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