US President Barack Obama on Tuesday offered the Philippines a warship as part of a $250-million aid package to Southeast Asian allies worried about Chinese efforts to control the South China Sea.

Obama made the pledges aboard the Philippine Navy's flagship, shortly after arriving in Manila for a summit of Asia-Pacific leaders to also be attended by Chinese President Xi Jinping.

"My visit here underscores our shared commitment to the security of the waters of this region and to the freedom of navigation," Obama said as he announced the assistance.

The offers were aimed at reassuring allies that the United States was committed to maintaining security in the region's waters, following Chinese artificial island building in parts of the South China Sea.

China claims nearly all of the sea, a strategically vital waterway home to some of the world's most important shipping routes.

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

China's building of artificial islands close to the Philippines prompted the US to deploy a missile destroyer and B-52 bombers to the area in recent weeks.

China was almost certain to react angrily to Obama's announcement, as it insists the United States has no right to involve itself in disputes over waters that are far away from US coasts.

China had also repeatedly called for the annual Asia-Pacific Economic Cooperation summit, which starts on Wednesday, to focus exclusively on trade and not be distracted by the rows.

In Beijing just before the US announcement, Chinese vice foreign minister Liu Zhenmin warned the other claimants that China could take control of the islands they occupy.

"The Chinese government has the right and the ability to recover the islands and reefs illegally occupied by neighbouring countries," Liu said.

"But we didn't do that, we exercised maximum restraint."

- 'Ironclad' support -

The Philippines, which has one of the weakest militaries in Asia and is the most vocal critic of China's actions in the sea, will receive the most support under the US package.

Obama said the Philippines would get a decommissioned US Coast Guard cutter to be turned into a new warship that will "bolster the navy's ability to conduct long-endurance patrols".

He said the Philippines would also get a research vessel to help map its territorial waters, while vowing US commitment to defend its longtime ally was "ironclad".

The Philippines will receive a record $79 million in assistance to bolster maritime security this financial year, the biggest recipient in Southeast Asia, the White House said.

"This will be a significant contribution for our maritime security capability," Philippine defence department spokesman Peter Galvez told AFP.

Vietnam, a former US enemy that has also spoken out strongly against China's regional assertiveness, will get $40.1 million in aid over this financial year and next, according to a White House statement.

Indonesia, which is not a claimant but has asked China to clarify its position in the sea, will get nearly $20 million to help "protect its maritime areas".

Malaysia, where Obama will travel to on Friday for another regional political summit, will receive $2.5 million worth of maritime security aid.

The Philippines and Vietnam also signed on Tuesday in Manila a strategic partnership to deepen security ties, cementing an alliance built partly on their concerns over China.

- Summit sidetracked -

Obama will on Wednesday meet Xi at the start of the APEC summit, which groups leaders from 21 Pacific rim economies that account for more than half of the global economy.

While the two-day summit is meant to focus on fostering trade unity, the annual event is often sidetracked by other global issues.

This year it will be held under the global shadow cast by last week's rampage in Paris claimed by the Islamic State group that killed at least 129 people.

Obama, Xi and a host of other leaders arrived in Manila on Tuesday from Turkey, where they attended a summit of the Group of 20 top economies that also focused heavily on IS and how to destroy the jihadist network.

Philippine authorities, which had already deployed more than 20,000 police and soldiers for the summit, said security had been ratcheted up even higher because of the Paris attacks.