Police and protesters clashed Thursday outside a gathering of Asia-Pacific leaders who were meeting for a summit dominated by a US-China tussle for regional influence.

The annual 21-member Asia-Pacific Economic Cooperation (APEC) gathering -- hosted this year by the Philippines -- is meant to forge trade unity but often finds itself sidetracked by other events.

This week US President Barack Obama has sought to bolster allies locked in a territorial row with China over the South China Sea, which is home to some of the world's most important shipping lanes.

And despite the Philippines undertaking its biggest security operation for the summit with more than 20,000 police and soldiers deployed, anti-APEC protests flared on Thursday morning close to the summit venue.

Police with full riot gear, including helmets, shields and wooden batons, fired water cannon at hundreds of protesters who tried to break through barricades about one kilometre (around half a mile) from the summit venue.

However, despite on-and-off scuffles throughout Thursday morning, there was no major violence and police allowed the protesters -- who mainly hailed from left-wing groups -- to continue their rally behind the barricades.

Smaller nations with overlapping claims to the South China Sea, including the Philippines, have been rattled by Beijing's increasing assertiveness in the waters in recent years.

These actions have included a spate of island building on disputed reefs and shoals, and many Asian nations have been looking to Washington for support.

- Chinese anger -

China had been hoping that the long festering territorial dispute would not surface at the trade gathering.

But those hopes were dashed when US president Barack Obama flew into Manila on Tuesday and announced more than $250 million in maritime aid for those Southeast Asian allies.

He also offered a warship to the Philippines, one of Beijing's most vocal critics in the dispute.

On Wednesday, shortly before the trade talks got under way, Obama repeated Washington's demand that China halt any further land reclamation and militarisation.

"We discussed the impact of China's land reclamation and construction activities on regional stability," Obama told reporters after meeting Philippine President Benigno Aquino.

"We agree on the need for bold steps to lower tensions, including pledging to halt further reclamation, new construction, and militarisation of disputed areas in the South China Sea."

Vietnam also signed a strategic partnership this week in Manila with the Philippines, deepening security ties partly in response to Chinese assertiveness in the sea.

Chinese President Xi Jinping has so far studiously avoided bringing up the dispute at the talks in Manila.

But officials in Beijing reacted with anger to Obama's efforts to bolster US allies, while insisting China's construction work in the contested areas was "lawful, justified and reasonable".

"If there is something that should stop, it is the United States should stop playing up the South China Sea issue, stop heightening tensions in the South China Sea," foreign ministry spokesman Hong Lei said in Beijing.

APEC members the Philippines, Vietnam, Malaysia, Brunei and Taiwan have rival claims to parts of the sea, which is also believed to sit atop vast oil and gas resources.

Global terrorism concerns were another hot topic in the Philippines this week, following a series of attacks in Paris last week claimed by the Islamic State group that claimed 129 lives.

The APEC leaders were due to issue a statement on Thursday calling for increased global co-operation to fight terrorism, according to a draft of their end-of-summit communique seen by AFP.

"We strongly condemn all acts, methods, and practices of terrorism in all their forms and manifestations," the draft statement said.

"We will not allow terrorism to threaten the fundamental values that underpin our free and open economies.

"We stress the urgent need for increased international cooperation and solidarity in the fight against terrorism."

Obama and the leaders of the Southeast Asian nations attending the Manila trade talks will proceed onto Malaysia for another regional summit.

The South China Sea row, the spectre of terrorism and the US-led Trans Pacific Partnership free trade pact are likely to again be the main subjects of discussion in Malaysia.