US President Barack Obama has put a firm focus on the South China Sea dispute, as an economic summit of Asia Pacific nations begins in Manila.
Mr Obama said China must stop land reclamation in the disputed waters, a day after committing monetary and naval assistance to the Philippines, which has competing claims with China.
The territorial dispute is not officially on the agenda of the summit but is expected to overshadow it.
China says its dredging work is legal.
The land reclamation, which began in late 2013, has turned submerged reefs into islands. China has said it has “no intention to militarise” those islands.
‘Defence of our ally’
China’s President Xi Jinping is also in Manila for the Asia Pacific Economic Cooperation (Apec) summit of about 20 heads of state and governments.
Mr Obama landed in Manila on Tuesday, but then immediately boarded a US-donated Philippines navy frigate that operates around the Spratly Islands, which are claimed by both the Philippines and China.
“We have a treaty obligation, an iron-clad commitment to the defence of our ally the Philippines,” he said on board.
“My visit here underscored our shared commitment to the security of the waters of this region and to freedom of navigation.”
He did not mention China, but announced that two more vessels would be transferred to the Philippines along with a $250m (£164.5m) package to enhance regional maritime security.
Analysis: Rupert Wingfield-Hayes, BBC News, Manila
America has been taken aback by the speed at which China has built new artificial islands and runways on reefs in the South China Sea over the last year-and-a-half.
Now Washington is responding – sending Navy ships and even B-52 bombers into the area in recent weeks.
The message is clear, the US will not allow China to proceed unchallenged with a take-over of one of the busiest and most strategic bits of water in the world.