MANILA :  The Philippines told the US on Tuesday it was quitting a pact key to their historical military alliance, triggering a six-month countdown to the deal’s termination, Manila said.

Philippine President Rodrigo Duterte made the move after voicing anger over American authorities cancelling the visa of the official who led his internationally condemned drug war.

This comes after Duterte has, since taking power in mid-2016, repeatedly made unrealised threats to shrink or even sever his nation’s ties with its former colonial master and most important military ally.

The 1998 Visiting Forces Agreement (VFA) is the legal framework for the presence of US troops on Philippine soil and is central to the two nations´ hundreds of annual, joint military exercises, which are a major component of their deep military ties.

The pact requires a 180-day notice to quit, which will likely set off a period of negotiation between the two countries.

“The Deputy Chief of Mission of the Embassy of the United States has received the notice of termination of the Visiting Forces Agreement,” Philippine foreign affairs secretary Teodoro Locsin wrote on Twitter.

Duterte’s spokesman, Salvador Panelo, told a press briefing the notice was sent on Tuesday.

US embassy officials did not immediately reply to requests for comment.

The VFA is divise in the Philippines, with left and nationalist critics long arguing it guarantees special treatment of American service members accused of crimes.

But defenders of the decades-old agreement say ending it could both degrade the Philippines’ ability to defend itself and undermine American moves against Beijing’s rise, particularly in the disputed South China Sea.

Locsin, an advocate of the pact, told lawmakers last week it was fundamental to the US alliance, which had provided tens of millions of dollars in support, equipment and training for the Philippine armed forces.

“Terminating the VFA will negatively impact its (the Philippines´) defence and security,” Locsin said.

“Our contribution to regional defence is anchored on our military alliance with the world´s last superpower,” he added.

Duterte first warned he would scrap the deal in 2016 and then repeated that threat in January in a speech.

Duterte’s speech last month came after Ronald Dela Rosa, the former national police chief who is now a senator, said the US had cancelled his visa but did not tell him why.

Philippine officials view the cancellation as a sanction for Dela Rosa’s leadership of the campaign in which police have killed thousands of alleged pushers and users.

Since the visa issue surfaced, Duterte has barred his cabinet from going to the US and has turned down Trump’s invitation to a southeast asian summit set for March in Las Vegas.

Duterte bristles at any Western criticism of his signature policy, which public opinion surveys say is widely supported by Filipinos despite the killing.