US President Donald Trump on Wednesday brushed aside the decision of Philippine President Rodrigo Duterte to end a decades-old military agreement with the United States, saying he did not really mind and it would save money.

Duterte announced the termination of the two-decade-old Visiting Forces Agreement (VFA) on Tuesday, a move US Defense Secretary Mark Esper called "unfortunate". The decision will take effect in 180 days.

Asked whether he would try to persuade Duterte to reconsider, Trump told reporters at the White House: "Well I never minded that very much, to be honest. We helped the Philippines very much. We helped them defeat ISIS ... I don't really mind if they would like to do that, it will save a lot of money. My views are different from others."

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Trump said he had "a very good" relationship with Duterte and added, "we'll see what happens."

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

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

Filipino and US soldiers march during the closing ceremony of a joint US-Philippine military exercise in 2018 [File: Mark Cristino/EPA]

Ending the VFA could also hurt Washington's future interests in maintaining an Asia-Pacific troop presence amid friction over the presence of US personnel in Japan and South Korea and security concerns about China and North Korea.

Duterte has said the US uses the pacts to conduct clandestine activities such as spying and nuclear-weapons stockpiling, which he said 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.

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

Philippine President Rodrigo Duterte speaking during his fourth State of the Nation Address at the Philippine Congress in Quezon City, Metro Manila, Philippines [Eloisa Lopez/Reuters]

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