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

The 2014 Enhanced Defense Cooperation Agreement, known as EDCA, gave US forces access to five bases in the Philippines. It has been expanded to nine, but the locations of the four additional bases have not been revealed as the government consults with local officials.

It has been widely reported that two of the sites will be in the northern province of Cagayan, less than 400 kilometres (250 miles) from self-ruled Taiwan, which China sees as part of its territory.

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

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

"The two countries will announce as soon as they can (the locations)," said US Air Force Secretary Frank Kendall at a joint press conference with Galvez and other officials.

Galvez and Kendall were at Basa Air Base, north of the capital Manila, where the United States is investing $24 million in upgrading the 2.8 kilometre runway.

Basa is one of the five bases originally included in the EDCA.

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

The pact stalled under former president Rodrigo Duterte, who favoured 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".

But Kendall said it was for the "good of the region".

"The defence treaty with the Philippines... is about mutual protection and peace and security in the region in general," Kendall said.