Here are some facts on the South China Sea, the maritime rules governing its waters, and major players embroiled in disputes within it.
The South China Sea covers an area of more than 648,000 sq miles (1.7 million sq km), containing more than 200 mostly uninhabitable small islands, rocks and reefs. It borders China and Taiwan to the north, Vietnam to the west, Malaysia, Brunei, Indonesia, and Singapore to the south and southwest, and the Philippines to the east.
The shortest route between the Pacific and Indian Oceans, it has some of the world's busiest shipping lanes. More than half the globe's oil tanker traffic passes through it. Most shipping is of raw materials, such as crude oil from the Gulf to East Asian countries. The sea holds valuable fishing grounds, and as-yet largely unexploited oil and natural gas fields.
Six parties are involved in a complex set of historically based territorial disputes in the sea -- Brunei, China, Malaysia, the Philippines, Taiwan and Vietnam. China's claims, the broadest, cover all of the Spratly and Paracel islands and most of the South China Sea.
China's military occupies all of the Paracels, and some nine reefs in the Spratly Islands, including Johnson South Reef, Hughes Reef and Subi Reef.
Vietnam occupies dozens of Spratly atolls and reefs and has military bases on several more.
Taiwan holds Itu Aba island and Ban Than Reef in the Spratlys. Former president Chen Shui-bian visited Itu Aba in 2008 with a naval flotilla. Taiwan has built an airport there.
Malaysia has built an air strip and diving resort on Layang Layang, also known as Swallow's Reef. The Malaysian navy maintains a base here too. The other atolls it occupies are Ardasier Reef, Marivales Reef, Erica Reef and Investigator Shoal.
The Philippines occupies several Spratly Islands, most significantly Thitu, which it renamed Pagasa (Hope).
Brunei occupies none of the islands.
The biggest military skirmishes occurred in 1974, when China attacked and captured the western Paracels from Vietnam, and in 1988, when China and Vietnam fought a brief naval battle near the Spratly reefs, in which more than 70 Vietnamese sailors died.
Vietnam recently ordered six Kilo-class diesel submarines from Russia as part of a major arms purchase that analysts see as an attempt to counterbalance China's growing naval reach.
Vietnam and China have competing claims over undeveloped oil and gas blocks. Businessmen and diplomats say China has pressured foreign firms in deals with Vietnam not to develop those blocks.
In 2007, BP Plc (BP.L) halted plans to conduct exploration work off the southern Vietnamese coast due to the territorial dispute between Hanoi and Beijing.
Vietnamese fishing boats are frequently halted and the fishermen detained by Chinese patrol vessels in disputed waters, to Hanoi's displeasure. In many cases, reports say they are freed only after the Vietnamese government pays China.
In 2002, the member states of the Association of South East Asian Nations (ASEAN) and China signed a non-binding Declaration on the Conduct of Parties in the South China Sea, urging the claimant states to exercise restraint and avoid activities that might escalate tension, such as construction of military facilities and holding war games.
Most claimants are developing tourism on or around some of the islands they hold to bolster their claims.
The 1982 United Nations Convention on the Law of the Sea allows coastal states to establish sovereignty over two areas: 1. Territorial seas -- adjacent waters spanning a maximum of 12 nautical miles from their coastlines, including the coastline of offshore islands, and 2. Exclusive Economic Zones (EEZ) -- extending 200 nautical miles from the coast.
UNCLOS says overlapping claims should be resolved through ad hoc arbitration or submission to international courts.
THE UNITED STATES:
The U.S. has not ratified UNCLOS, objecting to a clause on seabed mineral exploration. But when accused by China of illegal trespass, it has referred to its provision for states to conduct intelligence-gathering activities in EEZ's. U.S. surveillance aircraft and ships have long conducted surveys in the sea. The country's main security concern in the area is keeping open the sea routes that are vital for commercial shipping and warships.
China has signed and ratified UNCLOS. It says all the islands have been Chinese since ancient times.
Malaysia says that its claims to territories and maritime areas in the South China Sea are in accordance with principles of international law and as depicted in a map it published in 1979 which defined the country's continental shelf boundaries.
In 1978, former president Ferdinand Marcos issued a decree claiming the entire territory as part of the Philippines, redrawing the country's map. Manila is a signatory to UNCLOS and has passed a law asserting its claims on the Spratlys.
Taiwan claims the Spratly, Paracel and Pratas islands in its constitution. VIETNAM: Hanoi has ratified UNCLOS. In 2009, Vietnam and Malaysia presented a joint submission to UNCLOS on their claims which underlines the point that while China prefers to deal with the competing claimants on a bilateral basis, others have been pushing for a multilateral approach to the disputes.
Brunei claims part of the South China Sea as its Exclusive Economic Zone, a section of which includes Louisa Reef.
(Writing by Ben Blanchard; Editing by Alex Richardson)