The Philippines will send military patrols and survey vessels to a large underwater land mass in the Pacific Ocean to assert its rights over the area after Chinese ships were spotted there.

The vessels will travel to Benham Rise, 250km off the Philippines' northeastern coast, Defence Secretary Delfin Lorenzana announced on Tuesday.

"We will send our patrols and then we will study what we can put there to exploit also the resources," Lorenzana told reporters.

"We will continue to study what will be the best way to develop that area for our needs. We are entitled to all the economic benefits of Benham Rise," he added.

READ MORE: Duterte tells China, Benham Rise sea territory 'is ours'

The Philippine foreign ministry said on Tuesday that there was no question - and no rival claim - over the 13-million hectare undersea region rich in minerals and biodiversity.

"It is indisputable because no other country has an overlapping claim there," Charles Jose, a foreign ministry spokesman, said in a statement.

"So, as a country that exercises sovereign rights and jurisdiction, we are the only one that has a sole and exclusive right to explore and exploit the national resources in Benham Rise. It is our responsibility to protect it."

In 2012, a UN commission approved the Philippines' territorial claim to the area.

'Sovereign rights'

Justice Antonio Carpio, a Supreme Court member, told the Manila-based Inquirer.net that the UN convention reserves the "sovereign rights" over the area to the Philippines, allowing it exclusive rights to explore oil, gas and other minerals.

But he also said that other states, including China, have the right to conduct fishery research because the fish in the Philippines' extended continental shelf "belongs to mankind".

The Philippine military said it had monitored Chinese survey ships over Benham Rise during the past six months of 2016.

It was not clear what the survey ships were doing there, but Lorenzana said that he had received information the Chinese were "looking for a place to put submarines".

On Monday, President Rodrigo Duterte called for "structures" to be erected in the area to denote Philippine jurisdiction, and he told the navy to "go there and tell them straight that this is ours".

The territory is not part of the South China Sea, a key shipping lane that Beijing claims almost entirely, despite the Philippines, Brunei, Malaysia, Taiwan and Vietnam also having claims.