Philippine President Rodrigo Duterte on Tuesday announced the termination of a two-decade-old Visiting Forces Agreement with the United States (VFA), delivering on threats to downgrade an alliance important to U.S. interests.

The mercurial Duterte, who has clashed with the former colonial ruler over several issues, decided to pull the plug on the two-decade troop rotation pact to enable the Philippines to be more independent in its relations with other countries, spokesman Salvador Panelo said.

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

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

It would also limit Philippine access to U.S. training and expertise in tackling Islamist extremism, natural disasters and maritime security threats.

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

The U.S. Embassy in Manila called it "a serious step with significant implications."

"We will carefully consider how best to move forward to advance our shared interests," it said in a statement.

Given the importance of the alliance with the Philippines in broader U.S. strategy, Washington still hopes the decision will be reversed or delayed before it takes legal effect in 180 days.

In Washington, a senior Trump administration official voiced disappointment over the decision.

"The United States shares a long history with the government and people of the Philippines and recognizes that regional and global security is best served through the strong partnership that is enabled by the Visiting Forces Agreement," the official said.

"We will continue to work with our Philippine government partners to strengthen this relationship in a way that benefits both our countries."

The VFA is important to the overall U.S.-Philippines alliance and sets out rules for U.S. soldiers to operate in the Philippines. It underpins what Washington has called an "ironclad" relationship despite Duterte's complaints about U.S. hypocrisy, ill-treatment and ageing weapons.

Duterte says the United States uses the pacts to conduct clandestine activities like spying and nuclear weapons stockpiling.

Ending the VFA could hurt Washington's future interests in maintaining an Asia-Pacific troop presence amid friction over the presence of U.S. personnel in Japan and South Korea.

Senate's pushback

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

Those cover dozens of annual military training exercises and broaden the access of U.S. forces and equipment to the Philippines, as well as bind the two countries to defend each other from external aggression.

Duterte, who favors closer ties with China and Russia, pressed ahead with the termination despite a Senate hearing on the VFA last week during which his senior generals and defense and foreign ministers spoke in favor of it.