He said he made the promise during a meeting last night with the families of those killed in a shopping mall fire in the southern city of Davao.
"I assured them... that the truth will - let the truth come out," Duterte said. "That is what they are asking for. Just the truth of what happened."
The justice and labour departments have ordered separate investigations into Saturday's blaze.
Justice Secretary Vitaliano Aguirre has said his office would investigate the fire with a view to bringing criminal charges.
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At a Davao hospital today families of the dead waited in a silence broken only by occasional sobs as government workers tried to identify the charred remains before releasing them to relatives.
Social welfare officers said that so far, five bodies had been turned over.
Rhen Muyco recalled the last message his 25-year-old daughter Renzi Nova sent the family as the fire raged Saturday.
Labour Secretary Silvestre Bello said today his office was launching an inquiry separate from the justice department's investigation.
"We just want to find out the cause of the fire and if there was compliance with safety and health standards," he told AFP.
The Associated Labor Unions said the high death toll and the extent of the blaze suggested that rules on fire exits, sprinkler systems and other safety measures had not been followed.
Mall administrators have denied that fire exits were inadequate or blocked.
The fire was just one of a series of tragedies that turned the usually festive Christmas season in the Philippines into one of grief for many.
At least 240 people were killed, with over a hundred still missing, when Tropical Storm Tembin struck the country's main southern island of Mindanao on Friday, causing floods and landslides throughout the weekend.
On Monday 20 people were killed in a road accident in the northern Philippines as they headed for a traditional Christmas mass.
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