Last month, Marcos granted the US access to four sites, on top of five existing locations under the 2014 EDCA, which comes amid China's increasing assertiveness in the South China Sea and towards self-ruled Taiwan.

"There are four extra sites scattered around the Philippines. There are some in the north, there are some around Palawan, there are some further south," Marcos told reporters at the sidelines of the Philippine army's founding anniversary.

The EDCA allows US access to Philippine bases for joint training, pre-positioning of equipment and building of facilities such as runways, fuel storage and military housing, but it is not a permanent presence.

The Philippines and the US would announce the locations of the bases soon, Marcos said, adding the sites would boost the country's ability to defend the "eastern side" of its largest island, Luzon. Luzon is the closest main Philippine island to Taiwan.

China's foreign ministry on Wednesday reiterated its stance that the US side was increasing tensions by strengthening its military deployments in the region, adding that countries should be "vigilant" and avoid being used by the US.

"We generally believe that defense cooperation between countries should be conducive to regional peace and stability, and should not be aimed at third parties or harm the interests of third parties," spokesperson Wang Wenbin told reporters at a regular news briefing.

A former Philippine military chief has publicly said the US has asked for access to bases in Isabela, Zambales and Cagayan, all on the island of Luzon, facing north towards Taiwan, and on Palawan in the southwest, near the disputed Spratly Islands in the South China Sea.

Some leaders of local governments at the potential EDCA sites have opposed Marcos' decision, worried they would be dragged into a conflict if one arose between the US and China over Taiwan.

But Marcos said his government had discussed with them the importance of the expanded US access and "why it will actually be good for their provinces."

Washington has committed US$80 million worth of infrastructure investments at the five existing sites: the Antonio Bautista Air Base in Palawan, Basa Air Base in Pampanga, Fort Magsaysay in Nueva Ecija, Benito Ebuen Air Base in Cebu and Lumbia Air Base in Mindanao.

Speaking before Philippine troops, Marcos told them to be vigilant as the external threat to his country's security was becoming more "complex" and "unpredictable."

"Be vigilant against elements that will undermine our hard earned peace, our hard earned stability. Continue to improve relations with your counterparts overseas," Marcos said.

Without giving specifics, Marcos said he was aware of an "emerging threat" to his country's territory, which he said would require "adjustments in our strategy."

"The external security environment is becoming more complex. It is becoming more unpredictable," Marcos said.

The Philippines and the US will carry out their biggest ever joint military drills next month.

This year's Balikatan or shoulder-to-shoulder drills also come against the backdrop of what the Philippines calls "aggressive" Chinese actions in the South China Sea.

The drills, held from April 11 to 28, will see 17,600 participants from both sides, including around 12,000 from the US, and "live fire exercises into the water" for the first time, according to Col. Michael Logico, director of the Philippine military's training center and spokesperson for the event.

Previously, the biggest joint drills were undertaken in 2015, with more 11,000 troops participating.

The Balikatan exercises are to be carried out across several provinces, including Palawan, which is near the South China Sea.

They will also see some 111 participants from the Australian defense force, though their participation will be limited to smaller land-based exercises.