China rejected Philippine complaints on Wednesday about Chinese survey vessels operating in a gas-rich area of Manila's exclusive economic zone, and has lodged a separate complaint about the detention of Chinese workers, as tensions persist.
Philippine President Benigno Aquino said over the weekend that two Chinese survey vessels had been sighted in part of the disputed South China Sea also claimed by the Philippines called Reed Bank.
In a statement faxed to Reuters, China's Foreign Ministry said that Reed Bank was Chinese territory.
"Survey activities being carried out by Chinese survey ships are appropriate, legal and beyond reproach," the ministry said.
China claims almost the entire South China Sea, believed to be rich in deposits of oil and gas resources. Brunei, Malaysia, the Philippines, Taiwan and Vietnam also claim the sea where $5 trillion of ship-borne goods pass every year.
Tensions in the South China Sea between the Philippines and China began in 2011 when Chinese patrol boats harassed a survey ship hired by Anglo-Filipino Forum Energy PLC which won a contact to explore the Reed Bank.
Another two to three Chinese ships have been seen regularly on patrol around Scarborough Shoal, another part of the disputed South China Sea.
Last month, Forum Energy PLC obtained an approval from the Philippine government to extend by one year its drilling plan for a natural gas project in Reed Bank. The company is now expected to complete by August 2016 drilling for its appraisal wells to assess the size of gas or oil discoveries.
Relations between Beijing and Manila have also been strained due to the detention of Chinese fishermen caught in waters claimed by both countries.
In a separate incident, the Chinese embassy in Manila lodged a complaint about the detention on Tuesday of more than 50 Chinese suspected of working illegally in the country.
The embassy is demanding that the authorities handle the case swiftly and fairly and treat those detained properly, the embassy said in a statement.
Philippine's immigration authorities said that the 55 Chinese had been picked up in raids across Manila as they were believed to working without the necessary permits.
Reporting by Ben Blanchard