Five men were arrested last week after they allegedly used drones and high-resolution camera equipment to record activities at Filipino air and naval bases -- including movements of government vessels supplying military garrisons in the disputed South China Sea.

A Chinese software engineer and two Filipino associates were also detained earlier in January for alleged spying on military and police camps -- allegations that were dismissed by the Chinese embassy in Manila.

"We are very disturbed by anyone conducting such espionage operations against our military," Marcos told reporters.

In a statement, National Security Adviser Eduardo Ano said the arrests underscore the need for "continued vigilance and proactive counterintelligence measures".

The series of arrests comes as maritime tensions grow between the Philippines and China over contested reefs and waters in the South China Sea.

Beijing claims most of the strategic waterway despite an international ruling that its assertion has no legal basis.

Chinese authorities have yet to comment on the latest arrests.