Philippine officials accused China on Wednesday of expanding reclamation work in disputed waters of the South China Sea, as the United States again called for restraint in the territorial conflict.

Philippine and US officials spoke as the longstanding allies ended a two-day strategic dialogue in Manila, reaffirming defence and economic cooperation.

"The Chinese activities in the West Philippine Sea (South China Sea) continue to be a serious concern, arising from reports of greater development in reclamation," Philippine defence undersecretary Pio Lorenzo Batino told reporters.

Batino refused to elaborate on the progress of the reclamation, saying only: "It's very serious. It has grown."

Philippine foreign undersecretary Evan Garcia described the Chinese reclamation work as "massive" and a violation of an agreement among South China Sea claimants not to build new structures until a binding code of conduct is in place.

"It is not helpful in finding a way forward. It is not an example of what everyone would understand as self-restraint," Garcia said.

The Philippines last year accused China of blasting sand and rocks to reclaim areas around reefs in the sea. Facilities under construction reportedly include an airstrip.

The United States has "laid out a persuasive case for restraint" in the South China Sea, said assistant secretary of state Daniel Russel, calling the territorial disputes an "ongoing concern".

"We believe bigger nations can't bully the small," he said.

"We have a huge interest in stable, healthy, constructive bilateral relations with China," Russel said, but the US was also concerned about "behaviour that raises tensions, behaviour that raises questions about China's intentions".

US assistant defence secretary David Shear said Washington strongly supports Philippine efforts to modernise its military, which is one of the weakest in the region.

He said the US had provided $300 million in military-related assistance since 2001 and would provide another $40 million in 2015.

China's claim to much of the South China Sea conflicts with claims by the Philippines as well as those of Brunei, Malaysia, Taiwan and Vietnam.

The Philippines last March, filed a formal plea to the United Nations challenging China's claims.