The longtime treaty allies agreed in February to expand cooperation in "strategic areas" of the country as they seek to counter China's growing assertiveness over self-governed Taiwan and its building of bases in the South China Sea.

The 2014 Enhanced Defense Cooperation Agreement (EDCA), gave US forces access to five bases in the Philippines. 

It was expanded to nine, but the locations of the four additional bases were withheld until Monday while the government consulted with local officials.

The four sites had been assessed by the Philippine military and deemed "suitable and mutually beneficial", the Presidential Communications Office said in a statement.

It added that the bases would also be used for humanitarian and relief operations during disasters.

A US official confirmed that the locations announced by the palace were the new EDCA sites. 

Three of the sites are in the northern Philippines, including a naval base and airport in Cagayan province and an army camp in the neighboring province of Isabela, the statement said.

The naval base at Cagayan province's Santa Ana is about 400 kilometers from Taiwan. 

Another site will be on Balabac Island, off the southern tip of Palawan Island, near the South China Sea.

Cagayan Governor Manuel Mamba has publicly opposed having EDCA sites in his province for fear of jeopardizing Chinese investment and becoming a target in a conflict over Taiwan.

But Philippine acting defense chief Carlito Galvez told reporters recently the government had "already decided" on the sites and that Mamba had agreed to "abide with the decision".

The agreement allows US troops to rotate through the bases and also store defense equipment and supplies at them. 

The pact stalled under former president Rodrigo Duterte, who favored China over the country's former colonial master. 

But President Ferdinand Marcos, who succeeded Duterte last June, has adopted a more US-friendly foreign policy and has sought to accelerate the implementation of the EDCA.

Beijing has been critical of the agreement, which its embassy in the Philippines said recently was part of "US efforts to encircle and contain China through its military alliance with this country".

The Chinese embassy did not immediately respond to a request for comment.

Largest joint war games

In the meantime, the Philippines and the US are set to hold their largest ever joint military exercises this month, as the longtime allies seek to counter China's growing regional influence.

A total of 17,680 soldiers will take part in the annual drills, which for the first time will include live-fire exercises in the disputed South China Sea and a simulated defense of a tiny Philippine island nearly 300 km south of Taiwan.

The countries will also stage an amphibious landing on the western island of Palawan, the closest Philippine landmass to the Spratly Islands, a flashpoint for Beijing and Manila.

"Any armed forces has the right to conduct military exercises," said Colonel Michael Logico, the Philippine spokesman for the war games called Balikatan, which means "shoulder to shoulder" in Filipino.

"It's really part of our combat readiness," he said.

The exercises, scheduled for April 11-28, will involve more than 12,000 American, nearly 5,000 Filipino and 111 Australian soldiers – about twice as many as last year, Logico said.

The announcement came less than six weeks after Manila and Washington agreed to restart joint patrols in the South China Sea, and struck a deal to give US troops access to another four military bases in the Southeast Asian country.

The countries have been seeking to repair ties that were fractured under President Ferdinand Marcos's predecessor Rodrigo Duterte.

Beijing's growing assertiveness on Taiwan and its building of bases in the South China Sea have given fresh impetus to Washington and Manila to strengthen their partnership, which is underpinned by a mutual defense treaty.

One of the drills will involve military helicopters landing on Calayan island, off the northern tip of the main island of Luzon.

The maritime and coastal defense drill has been held in the past, but this will be the first time on Calayan.

It will also be the first time Philippine and US navy frigates will fire their weapons toward the South China Sea from waters off Zambales province, north of Manila, Logico said. 

Previous live-fire exercises were held on land.

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