MANILA—The Philippines notified the United States on Tuesday it would end a major security pact allowing American forces to train in the country, in the most serious threat under President Rodrigo Duterte to their 69-year treaty alliance.

Foreign Secretary Teodoro Locsin Jr. said in a tweet that 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. The termination would take effect after 180 days unless both sides agree to keep it.

Locsin signed the notice on the order 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.

U.S. Secretary of Defense Mark Esper said Tuesday in Brussels, Belgium, that he only received notice of Duterte's order on Monday evening and had not yet fully digested the details.

Esper called it “unfortunate.”

“I do think it would be a move in the wrong direction,” he said.

Esper said that when he visited the Philippines last November he thought the relationship was on a strong footing.

The U.S. Embassy in Manila acknowledged receipt of Manila’s notice and said Washington “will carefully consider how best to move forward to advance our shared interests.”

“This is a serious step with significant implications for the U.S.-Philippines alliance,” the embassy said in a statement. “Our two countries enjoy a warm relationship, deeply rooted in history. We remain committed to the friendship between our two peoples.”

In a Senate hearing last week, Locsin warned that abrogating the 1998 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, including criminal jurisdiction over erring American troops, instead of abrogating it. Philippine defense and military officials did not immediately issue any reaction to the government move.

Duterte threatened to terminate the security agreement after Washington reportedly canceled 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 bloody 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.

Duterte said in a speech late Monday that President Donald Trump has moved to save the agreement but added that he rejected the idea. He accused the U.S. of meddling in Philippine affairs, including seeking the release of opposition Sen. Leila de Lima, whom he has accused of involvement in illegal drugs. She has dismissed the allegation as a fabricated charge meant to muzzle dissent.

“America is very rude. They are so rude,” Duterte said.

Locsin outlined in the 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.

The accord, known by its acronym VFA, legally allows the entry of large numbers of American forces along with U.S. military ships and aircraft for joint training with Filipino troops. It specifies which country will have jurisdiction over American soldiers, who may be accused of crimes while in the Philippines, a sensitive issue in the former American colony.

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

A Filipino senator and former national police chief, Panfilo Lacson, said terminating the treaty would reduce the two allies’ 1951 Mutual Defense Treaty “to a mere paper treaty as far as the U.S. is concerned.”

Some Philippine senators have said the government decision to terminate the treaty, which the Senate ratified, should have the chamber’s consent.

U.S. State Department Assistant Secretary R. Clarke Cooper said in a telephone news conference Monday that abrogating the pact would put at risk more than 300 military engagements, including joint trainings, this year between the allies.

“All the engagements, all the freedom of navigation operations, all the exercises, all the joint training, having U.S. military personnel in port, on the ground, on the flight line, does require that we have a mechanism that allows that,” he said. “That’s why the VFA is so important.”

Terminating the VFA would affect the joint exercises and other activities 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.

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

China, the Philippines, Vietnam and three other governments have rival claims in 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. He has walked back on such threats but his government’s action on Tuesday is the most serious indication of his intent to set back military ties with the U.S.

Aside from threatening to end the VFA, Duterte has said he will ban some critical U.S. senators from entering the Philippines.

Duterte has also barred Cabinet officials from traveling to the U.S. and turned down an invitation by 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.