DAVAO CITY (UPDATED) - Presumptive President Rodrigo Duterte will pursue a "major rewriting" of the three-decade-old Constitution to shift to a federal-parliamentary form of government and ease foreign ownership restrictions on business, his spokesman said Tuesday.

Within his first six months in office, Duterte will ask Congress to pass a law for the election of members to a constitutional convention that will draft amendments to the charter, Peter Laviña told reporters here.

The proposed amendments would be submitted to a plebiscite by the 2019 mid-term elections and the first federal elections could be held by the end of his term in 2022, Laviña said.

"There will be a major rewriting of the constitution," Lavina said. "We have seen the failure of the presidential form [of government]."

Duterte plans to keep national defense, foreign affairs and customs under the central government and devolve the rest.

"He (Duterte) will be president towards that transition," he said.

Pursuing peace with Muslim and communist rebels will also be high on Duterte's agenda, Laviña said.

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"This will be a golden opportunity...the mayor is close with Muslim rebels and communist rebels," he said.

Lavina said Duterte was "friends" with some rebels, but this did not mean that he supported efforts to overthrow government.