There is still room for negotiations to replace a key military pact with the US that the Philippines has moved to scrap, a top Manila diplomat said Friday, voicing hope for the seven-decade-old alliance. President Rodrigo Duterte gave notice nearly three weeks ago of an end to the Visiting Forces Agreement (VFA), starting a six-month countdown to the deal's end.

The pact lays out the legal framework for US troops to be on Philippine soil and is central to hundreds of annual, joint military exercises, which are a major component of their deep defence ties.

"The VFA is something that obviously we have to still try to see where it can be polished or sort of like improved," Philippine ambassador to the US Jose Manuel Romualdez told a public forum.

"We are now in the process of trying to find ways and means to see how we either come out with something similar... still following the president's thinking about the sovereignty issue," he added.

Duterte ordered the VFA be terminated after accusing the United States of interference in his internationally condemned narcotics crackdown in which police have killed thousands of alleged dealers and users.

At the same time, he has repeatedly made unrealised threats to decrease or sever ties altogether with the Philippines' most important military ally and former coloniser.

Since coming to power in 2016, Duterte has set aside a once tense stand-off over China's extensive claims to the South China Sea, a strategic waterway, to court the Asian giant's trade and investment.

But at least one member of his administration has voiced concern over the impact of quitting the US deal.

Philippine Foreign Secretary Teodoro Locsin, an advocate of the pact, earlier told Congress the VFA was fundamental to the US alliance, which he said had provided tens of millions of dollars in support, equipment and training for the Philippine military.

Defenders have warned that ending the VFA could both degrade the Philippines' ability to defend itself and undermine Washington's moves against Beijing's rise, particularly in the South China Sea.