MANILA -- Philippine President Rodrigo Duterte has suspended the termination of a key military agreement with the U.S. by another six months, his foreign minister said late Monday, prolonging the uncertainties around one of the region's oldest defense alliances.

Duterte has now put off ending the pact for a third time. The move essentially extends the Visiting Forces Agreement, a crucial component of the alliance, which is being tested by China's expansion in the disputed South China Sea.

"The president conveyed to us his decision to extend the suspension of the abrogation of the Visiting Forces Agreement by another six months while he studies and both sides further address his concerns regarding particular aspects of the agreement," Foreign Affairs Secretary Teodoro Locsin Jr. said in a video message.

Locsin did not elaborate on the aspects of the deal still under review. The U.S. Embassy in Manila had yet to react to his announcement.

Duterte moved to abrogate the 1998 agreement, which facilitates the entry of U.S. troops into the Philippines for annual military drills, in February 2020 after Washington suspended the visa of one of his political allies.

Sailors man the rails aboard the amphibious assault ship USS Wasp as the ship arrives in Subic Bay, Philippines. (Photo courtesy of the U.S. Navy)

But the Philippine president suspended the termination twice by six months each time to let officials enhance the agreement. The pact was expected to end in August had Duterte decided to finally terminate it.

The Philippine ambassador to Washington, Jose Manuel Romualdez, said earlier this month that the new agreement submitted to Duterte had been "an improvement" following negotiations with the U.S. He did not elaborate.

Duterte's decision comes amid rising tensions in the South China Sea, where Manila and Beijing are locked in a territorial dispute. The Philippine military has recently reported spotting dozens of Chinese vessels within Manila's exclusive economic zone, triggering a flurry of diplomatic protests from the Philippines.

Romualdez called the Visiting Forces Agreement "important" as it puts into operation the 1951 Mutual Defense Treaty between the Philippines and the U.S. President Joe Biden's administration has assured Manila that an attack on a Philippine vessel in the South China Sea would trigger Washington's obligations under the treaty.

But Duterte has said the U.S. needs to "pay" if it wants to keep the military deal.

"It is a shared responsibility, but your share of the responsibility does not come free," the president said in a speech during a Philippine Air Force event in February.

Last year, he said Washington must deliver millions of doses of coronavirus vaccines if it wanted troops to remain in the Philippines.

An undisclosed number of U.S. military personnel are stationed on the southern Philippine Island of Mindanao, assisting in counterterrorism efforts.

The U.S. maintained military bases in the Philippines until the early 1990s. Subic Bay, which opens into the South China Sea, was the location of a former repair and supply base for the U.S. Navy's 7th Fleet. The site is now the Subic Special Economic and Freeport Zone. Clark Air Base was a U.S. Air Force facility that is now used by the Philippine Air Force. It is also home to the Clark Freeport and Special Economic Zone.

Washington last week said the U.S. would donate at least 80 million coronavirus vaccine doses globally by the end of June, and the Philippines will be one of the first beneficiaries. Biden said the vaccine donations come "with no strings attached."