A suicide attack in northern Mali Saturday that killed at least two UN troops will "not go unanswered", French President Francois Hollande vowed. "This heinous act cannot go unanswered," he said in a letter to Senegalese President Macky Sall, whose country provided the two UN peacekeepers killed. "France will be at your side to identify and arrest those behind this terrorist attack," Hollande said in the letter, according to a text made public during a visit the French leader was making to French Guiana. The two Senegalese soldiers were killed in Mali's northeastern city of Kidal when a suicide bomber ploughed his explosives-laden car into a bank they were guarding. They were part of the UN mission MINUSMA deployed in the country, in which France staged a military intervention in January to oust Islamic extremists.