"I accept the reality that the president is upholding the higher national interest," spokesman Harry Roque said, although he insisted he was expressing a personal opinion, not that of Duterte.

Duterte sparked controversy earlier this week for granting a surprise pardon to Lance Corporal Joseph Scott Pemberton, who was halfway into a 10-year sentence for killing Jennifer Laude in October 2014.

Laude was found dead in a motel room in the northern Philippines hours after she met the American at a bar in Olongapo city.

Roque said the pardon was linked to Duterte's desire to win swift access to a vaccine against a virus that has infected about a quarter of a million Filipinos, killing more than 4,000 of them.

"As we all know our president has emphasised the need for a vaccine," Roque told a virtual news conference.

"The grant of a pardon to Pemberton is in line with our president's desire that the Philippines should also benefit when Americans do develop a vaccine," he said, stressing he was stating his personal opinion.

Duterte's action has cleared all legal obstacles to the soldier's release after a local court ordered him freed early on the grounds of good behaviour.

Roque -- himself in quarantine after an aide tested positive for the virus -- previously served as a lawyer for the victim's family in the high-profile trial.

He rejected suggestions the Philippines was swapping its sovereignty for a coronavirus vaccine.

"The higher interest is saving lives," Roque said.