The Philippines notified the United States on Tuesday it intended to end a major security pact allowing American forces to train in the country in the most serious threat to their treaty alliance under President Rodrigo Duterte.

Foreign Secretary Teodoro Locsin Jr. said in a tweet Manila’s notice of termination of the Visiting Forces Agreement was received by the deputy chief of mission at the U.S. Embassy in Manila. He refused to provide other details on the drastic step "as a diplomatic courtesy."

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

In a Senate hearing last week, Locsin warned that abrogating the security accord with Washington would undermine Philippine security and foster aggression in the disputed South China Sea. U.S. military presence in the strategic waterway has been seen as a crucial counterweight to China, which claims virtually the entire sea.

Locsin proposed a review of the agreement to fix contentious issues instead of abrogating it. Philippine defense and military officials did not immediately issue any reaction to the government move.

The termination of the 1999 agreement would take effect 180 days after Washington received Manila’s notice but both could decide to keep the pact during the waiting period, Philippine officials said.

Duterte threatened to terminate the security agreement after Washington reportedly cancelled the U.S. visa of a loyal ally, Sen. Ronald dela Rosa, who was linked to human rights violations when he first enforced the president’s deadly anti-drug crackdown as the national police chief in 2016.

Thousands of mostly poor suspects have been killed under the anti-drug campaign Duterte launched when he took office in mid-2016, alarming the U.S. and other Western governments and human rights watchdogs.

Duterte gave the U.S. a month to restore dela Rosa’s visa, but U.S. officials have not publicly reacted to the Philippine leader’s demand.

Locsin outlined in last week’s televised Senate hearing what he said were the crucial security, trade and economic benefits the accord provides. The U.S. is a longtime treaty ally, a major trading partner and the largest development aid provider to the Philippines.

"While the Philippines has the prerogative to terminate the VFA anytime, the continuance of the agreement is deemed to be more beneficial to the Philippines compared to any predicates were it to be terminated," Locsin said.

The accord, known by its acronym VFA, took effect in 1999 to legally allow the entry of American forces along with U.S. military ships and aircraft to the Philippines for joint training with Filipino troops.

A separate defense pact subsequently signed by the allies in 2014, the Enhanced Defense Cooperation Agreement, allowed the extended stay of U.S. forces and authorized them to build and maintain barracks and warehouses and store defense equipment and weapons inside five designated Philippine military camps.

Terminating the VFA would affect more than 300 joint exercises and other activities this year with U.S. forces "which the Philippine military and law enforcement agencies need to enhance their capabilities in countering threats to national security," Locsin said.

The U.S. provided more than $550 million in security assistance to the Philippines from 2016 to 2019, Locsin said, adding that there may be a "chilling effect on our economic relations" if the Philippines draws down its security alliance with Washington.

American forces have provided intelligence, training and aid that allowed the Philippines to deal with human trafficking, cyberattacks, illegal narcotics and terrorism, Locsin said, citing how U.S. military assistance helped Filipino forces quell a disastrous siege by Islamic State group-aligned militants in southern Marawi city in 2017.

the U.S. military presence has also been a deterrent to aggressive actions in the disputed South China Sea, Locsin said.

China, the Philippines, Vietnam and three other governments have rival claims to the strategic waterway.

Duterte first threatened to abrogate the VFA in late 2016 after a U.S. aid agency put on hold funds for anti-poverty projects in the Philippines. The 74-year-old leader has walked back on such public threats before but his government’s action on Tuesday has been the most serious indication so far of his intent to set back military relations with the U.S.

Aside from threatening to take down the VFA, Duterte has said would ban some U.S. senators from entering the Philippines. He apparently was referring to American senators who sought to ban unspecified Philippine officials from entering the U.S. for their role in the continued detention of Philippine opposition Sen. Leila de Lima, a vocal critic of Duterte’s deadly campaign against illegal drugs.

Duterte has publicly accused de Lima of receiving money from drug traffickers and called for her detention. De Lima has dismissed the allegations as fabricated charges designed to muzzle dissent under Duterte.

Duterte has also barred his Cabinet officials from traveling to the U.S. and turned down an invitation by President Donald Trump to join a special meeting the U.S. leader will host for leaders of the Association of Southeast Asian Nations in March in Las Vegas, according to presidential spokesman Salvador Panelo.