Chinese trawlers are active in the South China Sea
A UN commission hoping to agree new maritime boundaries looks set to pit China against some of its neighbours.
China claims that a series of island chains in the South China Sea are part of its sovereign territory - but so do several other countries.
Most coastal states have to submit declarations on where they see their boundaries by 13 May.
A total of 48 nations have made full claims, and dozens more have made preliminary submissions.
"This is the sweep after which the maritime limits should be fixed... the final big adaptation of the world map," Harald Brekke, vice-chairman of the UN Commission on the Limits of the Continental Shelf, told Reuters news agency.
Under existing laws, a nation is allowed to exploit maritme resources up to 200 nautical miles from its shoreline.
But some nations are able to extend their claims as a result of their landmasses - or continental shelf - extending into the sea.
But the exact limits of who can use what have not been put on an internationally agreed map - until now.
"We are seeing many overlapping submissions," Mr Brekke said.
Territorial disputes between Japan and Russia, and between Britain and Argentina - over the Falkland Islands - have been highlighted by the process.
Russia has even used a mini-submarine to plant a flag on the seabed beneath the North Pole in 2007, an area that Denmark is also expected to claim.
But perhaps one of the most complicated areas to resolve is who owns what in the South China Sea, with China, the Philippines, Vietnam, Taiwan, Indonesia and Malaysia all having competing claims.
Chinese foreign ministry spokesman Ma Zhaoxu says the country has indisputable sovereignty over disputed South China Sea islands.
He says this jurisdiction also extends to what is below the seabed - which is important because the South China Sea has valuable oil and gas reserves.
China has recently become more assertive in pushing its territorial claims in the area, according to the BBC's Michael Bristow in Beijing.
It has formally told the UN not to consider a similar claim from Vietnam.
"[This] is a gross infringement upon China's sovereignty, sovereign rights and jurisdiction, thus illegal and invalid," said Mr Ma.
Other countries are not backing down - which means sorting out these competing claims will be a both complex and time-consuming, our correspondent says.