The West Philippine Sea (South China Sea) covers more than three million square kilometers), ringed by southern China, Taiwan, the Philippines, Borneo island, and mainland Southeast Asia.
Most of its hundreds of small islands, islets and rocks are uninhabited. The Paracel and Spratly chains contain the biggest islands.
The sea is the main maritime link between the Pacific and Indian oceans, giving it enormous trade and military value. Its shipping lanes connect East Asia with Europe and the Middle East.
Major unexploited oil and gas deposits are believed to lie under the seabed.
The sea is home to some of the world’s biggest coral reefs and, with marine life being depleted close to coasts, it is important as a source of fish to feed growing populations.
China and Taiwan both claim nearly all of the sea, while Vietnam, the Philippines, Malaysia and Brunei each have often-overlapping claims to parts of it.
Beijing’s argument is based largely on a decades-old Chinese map with a “nine-dash line” that approaches the coasts of other countries and outlines its claim.
Beijing and most other countries know it as the South China Sea. Hanoi calls it the East Sea and Manila officially refers to it as the West Philippine Sea.
China has held all of the Paracel islands since a conflict with South Vietnam in 1974 that left 53 Vietnamese troops dead.
Vietnam is believed to occupy or control more than 20 of the Spratly islands and reefs, the most of any claimant.
Taiwan has a garrison controlled by its coastguard on Itu Aba island, which is called Taiping in Chinese and is the largest in the Spratlys.
The Philippines occupies nine of the Spratlys, including Thitu island, the second largest. The Philippines has a military presence and civilians living on Thitu, which it calls Pagasa.
China occupies at least seven of the Spratlys including Johnson Reef, which it gained after a naval battle with Vietnam in 1988.
Malaysia occupies three of the Spratlys. The most significant presence is on Swallow Reef, called Layang Layang Island in Malaysia, where it has a naval post and a diving resort.
Brunei claims a submerged reef and a submerged bank in the Spratlys.
Aside from the 1974 battle for the Paracels, the only other major conflict occurred when Vietnam and China fought a naval battle on Johnson Reef in the Spratlys in 1988 that left 70 Vietnamese military personnel dead.
However, Chinese naval vessels have fired at other times on Vietnamese fishing boats in the area.
In June last year Vietnam passed a law proclaiming its jurisdiction over all of the Paracel and Spratly islands, triggering Chinese protests.
At about the same time China announced it had created a new city, Sansha, on one of the Paracel islands, to administer Chinese rule over its South China Sea domain.
In 1995, China began building structures on Mischief Reef, within the Philippines’ exclusive economic zone.
Tensions between the two nations rose in 2011 when Chinese vessels harassed a Philippine-chartered gas exploration vessel at Reed Bank.
The Philippines then accused Chinese military and paramilitary vessels of a campaign of intimidation within the country’s exclusive economic zone, including the occupation of Scarborough Shoal.
In January this year Manila asked a United Nations tribunal to rule that China’s claims were invalid. China refused to participate in the legal proceedings, which could take years.
In May, the Philippines said it had made an official protest at Chinese military vessels circling Filipino-occupied Second Thomas Shoal in the Spratlys.
The 10-member Association of Southeast Asian Nations (Asean) and China adopted a non-binding “declaration of conduct” in 2002 to discourage hostile acts. All sides agreed not to use threats or force to assert claims.
But China has since refused to turn it into a legally binding “code of conduct”.
The dispute exposed divisions at a gathering of Asean foreign ministers last year when the meeting host Cambodia, a China ally, rejected a Philippine push for the bloc to take a tough line against the Chinese.
With Brunei as host of Asean events in 2013, a sense of unity has been restored within the group. Asean foreign ministers issued a joint communique after their meeting on Sunday reaffirming their commitment to the 2002 declaration.