Bloomberg reported Tuesday, citing satellite images from US officials, that new land formations have emerged around the contested Spratly Islands in the sea, where a Chinese vessel with a hydraulic excavator was seen operating over the years.

"We are seriously concerned as such activities contravene the Declaration of Conduct on the South China Sea's undertaking on self-restraint and the 2016 Arbitral Award," the Philippine foreign ministry said late Tuesday in response.

The ministry added that other agencies have been asked to investigate the report.

Beijing claims almost all of the resource-rich waterway, through which trillions of dollars in trade passes annually. Rival claimants are the Philippines, Vietnam, Malaysia, Brunei and Taiwan.

China has ignored a 2012 ruling from a UN-backed tribunal that its claim is without basis.

In recent years it has built artificial islands on reefs in the disputed waters, and it has also constructed military facilities and airstrips.

The Philippines has repeatedly accused Chinese coast guard and maritime militia of harassing and attacking fishing boats and other vessels.

In the Spratlys, China occupies at least seven islands and rocks, militarising them with runways, ports, and radar systems.

The Bloomberg report said the fresh reclamations were taking place on Eldad Reef, Whitsun Reef, Sandy Cay, and Lankiam Cay.

The latter is 13 kilometres (eight miles) northeast of Philippine-occupied Loaita Island and 53.3 kilometres from Chinese-held Subi Reef.

About 450 kilometres to the east of Lankiam Cay is the Philippine island of Palawan, the nearest major landmass.

In Beijing, Chinese foreign ministry spokeswoman Mao Ning called the report "completely groundless".

"Not taking action on uninhabited islands and reefs of the South China Sea is a solemn consensus reached by China and ASEAN countries through actions and declarations by each party," she said, referring to the 10-member bloc.

"The development of China-Philippines relations currently has good momentum, and the two sides will continue to appropriately handle maritime issues through friendly consultation."

'Respect International Law' 

Manila filed a diplomatic protest last week against Beijing after a Chinese coast guard vessel in November "forcefully" seized debris from the Chinese rocket that was retrieved by a Philippine navy vessel.

Denying the use of force, the Chinese embassy in Manila said the handover came after a "friendly consultation".

The Philippine defence ministry also last week expressed "great concern" over the reported swarming of Chinese vessels in Iroquois Reef and Sabina Shoal, which Manila claims as its territory.

"(President Ferdinand Marcos') directive to the department is clear, we will not give up a single square inch of Philippine territory," acting defence secretary Jose Faustino said after the incident.

Marcos has insisted he will not let China trample on the Philippines' maritime rights, in contrast to his predecessor Rodrigo Duterte who was reluctant to criticise the superpower.

The US State Department spokesman this week expressed support to the Philippines on both incidents and called on China to "respect international law".

The Chinese embassy hit back on Tuesday, accusing Washington of using the dispute to "stir up troubles".

It acknowledged "differences" with Manila but did not address the alleged swarming incidents directly.