The daughter of outgoing Philippine President Rodrigo Duterte has indicated she will tackle drug issues as vice president, raising concern she may continue her father's hardline crackdowns on illegal drugs.

Sara Duterte took her oath on Sunday at a ceremony in the southern city of Davao, where she serves as mayor.

The ceremony was attended by Duterte's father and the president-elect, Ferdinand Marcos Jr. The father of the incoming president ruled the country with an iron fist until he was ousted in 1986.

Duterte was elected vice president by winning a record number of votes in the May 9 election. She and Marcos will take office on June 30.

She said children are facing a very complex future, the destruction brought about by drugs.

Her father's heavy-handed approach is said to have helped improve security in the country.

But the International Criminal Court is investigating the older Duterte over possible crimes against humanity, as he allowed police to kill drug suspects.

Human rights groups in the Philippines are expressing concern about whether the new vice president will continue her father's anti-drug campaign, and whether her position could affect the ICC investigation.