Tensions between Vietnam and China over the South China Sea came to a head in 2014 when Beijing moved an oil rig into waters claims by Hanoi

Philippine President Rodrigo Duterte said Monday he had agreed to allow Chinese surveillance ships into Filipino waters, contradicting his defence minister who described their presence as "very concerning".

Duterte also told reporters he did not want to have a "fight" with China over Benham Rise -- waters recognised by the United Nation as indisputably Philippine territory -- partly because he wanted Chinese economic help.

"They have no incursion because we have an agreement," Duterte told reporters when asked about the reported presence of Chinese surveillance ships at Benham Rise.

"Some people are just blowing it up. We previously agreed. It was a research ship. We were advised of it way ahead."

Duterte's comments came after his defence secretary, Delfin Lorenzana, said last week that Chinese surveillance ships had been seen in Benham Rise, which is believed to sit atop lucrative oil and gas deposits.

"The very concerning thing is they have several service ships plying this area, staying in one area sometimes for a month as if doing nothing. But we believe they are actually surveying the seabed," Lorenzana said.

"I have ordered the Navy that if they see this service ship this year, to start to accost them and drive them away."

Lorenzana said China may be "looking for a place to put submarines".

Duterte emphasised Monday the Philippines was set to enjoy billions of dollars in Chinese investments and grants, following his decision not to argue with China over another territorial dispute in the South China Sea.

"Let us not fight about ownership or sovereignty at this time because things are going great for my country," Duterte said in reference to China.

Benham Rise is an underwater landmass 250 kilometres (155 miles) off the east coast of the main island of Luzon.

In 2012, the United Nations Commission on the Limits of the Continental Shelf approved the Philippines' undisputed territorial claim to Benham Rise.

Chinese foreign ministry spokesman Geng Shuang said last week that although the UN had ruled in the Philippines' favour, this did not mean Benham Rise was part of its territory.

China and the Philippines have had a long-running dispute over competing claims in the South China Sea. Parts of that strategically vital waterway are also claimed by Brunei, Malaysia, Taiwan and Vietnam.

Duterte's predecessor, Benigno Aquino, had forcefully challenged China in diplomatic and legal circles over the South China Sea dispute, leading to a sharp deterioration in bilateral relations.

Duterte, who took office last year, has reversed that policy, preferring instead to placate China in return for hoped-for billions of dollars worth of investments and grants.