Philippine President Rodrigo Duterte formally terminated on Tuesday the two-decade-old Visiting Forces Agreement (VFA) with the United States, delivering on threats to downgrade an alliance important to US interests.

Foreign Secretary Teodoro Locsin Jr said in a social media post that Manila’s notice of termination was received by the deputy chief of mission at the US Embassy in Manila.

The termination will take effect after 180 days unless both sides agree to keep it.

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Locsin signed the notice on the order of Duterte, who has often criticised US security policies, while praising those of China and Russia despite the Philippine military’s close historic ties with its American counterpart.

US Defense Secretary Mark Esper called the decision "unfortunate" and said it would be a move in the wrong direction at a time when Washington and its allies were trying to press China to abide by "international rules of order" in Asia.

Duterte, who has clashed with the US over several issues, decided to pull the plug to enable the Philippines to be more independent in its relations with other countries, his spokesman Salvador Panelo said.

"The president will not entertain any initiative coming from the US government to salvage the VFA, neither will he accept any official invitation to visit the United States," Panelo said.

The decision, sparked by the revocation of a US visa held by a former police chief who led Duterte's bloody war on drugs, could complicate US military interests in the Asia-Pacific region as China's ambitions rise.

It would also limit Philippine access to US training and expertise in tackling armed groups in the southern island of Mindanao, natural disasters and maritime security threats.

Assault on sovereignty, judicial system

Panelo said Duterte's decision was a consequence of US legislative and executive actions that "bordered on assaulting our sovereignty and disrespecting our judicial system".

Given the importance of the alliance with the Philippines in broader US strategy, Washington hopes the decision can be reversed or delayed.

Esper told reporters travelling with him to Brussels for a NATO meeting he only received notification of the move late on Monday.

"We have to digest it. We have to work through the policy angles, the military angles. I'm going to hear from my commanders. But ... in my view, it's unfortunate that they would make this move," he said.

While he said he did not think the step was necessarily tied to China, it was a move "in the wrong direction as we both bilaterally with the Philippines and collectively with a number of other partners and allies in the region are trying to say to the Chinese: 'You must obey the international rules of order ...'."

The VFA is important to the overall US-Philippines alliance and sets out rules for US soldiers to operate in the Philippines.

'Ironclad' ties

It underpins what Washington has called an "ironclad" relationship despite Duterte's complaints about US hypocrisy, ill treatment and ageing weapons.

Duterte says the US uses the pacts to conduct clandestine activities such as spying and nuclear weapons stockpiling, which he says risk making the Philippines a target for Chinese aggression.

Some Filipino senators sought to block Duterte's move soon after news of it broke, arguing that without Senate approval he had no right to unilaterally scrap international pacts it had ratified.

"We must have a say on this important matter," said Senator Richard Gordon.

Some legislators are concerned that without the VFA, two other pacts would be irrelevant, namely the 2014 Enhanced Defence Cooperation Agreement made under the Obama administration, and a 1951 Mutual Defence Treaty.

Supporters of the agreements argue they have deterred Chinese militarisation in the South China Sea while $1.3bn of US defence assistance since 1998 has been vital in boosting the capabilities of under-resourced Philippine forces.

Philippine nationalists, however, say the US did nothing to stop China building islands in the South China Sea equipped with missiles and say the VFA is tilted in favour of the Americans, including the granting of immunity from prosecution for US servicemen.