China is claiming most of the territory as its own, a claim which is disputed by others.
Q&A: South China Sea dispute
Rival countries have wrangled over territory in the South China Sea for centuries - but a recent upsurge in tension has sparked concern that the area is becoming a flashpoint with global consequences.
What is the argument about?
It is a dispute over territory and sovereignty over ocean areas, and the Paracels and the Spratlys - two island chains claimed in whole or in part by a number of countries. Alongside the fully fledged islands, there are dozens of rocky outcrops, atolls, sandbanks and reefs, such as the Scarborough Shoal.
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Why are they worth arguing over?
Although largely uninhabited, the Paracels and the Spratlys may have reserves of natural resources around them. There has been little detailed exploration of the area, so estimates are largely extrapolated from the mineral wealth of neighbouring areas.
The sea is also a major shipping route and home to fishing grounds that supply the livelihoods of people across the region.
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Map of South China Sea
Who claims what?
China claims by far the largest portion of territory - an area defined by the "nine-dash line" which stretches hundreds of miles south and east from its most southerly province of Hainan.
Beijing says its right to the area goes back centuries to when the Paracel and Spratly island chains were regarded as integral parts of the Chinese nation, and in 1947 it issued a map detailing its claims. It showed the two island groups falling entirely within its territory. Those claims are mirrored by Taiwan.
Vietnam hotly disputes China's historical account, saying China had never claimed sovereignty over the islands before the 1940s. Vietnam says it has actively ruled over both the Paracels and the Spratlys since the 17th Century - and has the documents to prove it.
The other major claimant in the area is the Philippines, which invokes its geographical proximity to the Spratly Islands as the main basis of its claim for part of the grouping.
Both the Philippines and China lay claim to the Scarborough Shoal (known as Huangyan Island in China) - a little more than 100 miles (160km) from the Philippines and 500 miles from China.
Malaysia and Brunei also lay claim to territory in the South China Sea that they say falls within their economic exclusion zones, as defined by UNCLOS - the United Nations Convention on the Law of the Sea.
Brunei does not claim any of the disputed islands, but Malaysia claims a small number of islands in the Spratlys.
This handout photo taken on April 2, 2015 by satellite imagery provider DigitalGlobe shows a satellite image of what is claimed to be an under-construction airstrip at Fiery Cross Reef
Satellite imagery shows China building an airstrip in the Spratlys
A Filipino protester holds a child as they join a rally outside the Chinese consulate at the financial district of Makati, south of Manila, Philippines on Tuesday, 22 April 2014
The Philippines accuses China of strengthening its military presence in the South China Sea
Anti-China protesters rally in front of the statue of King Ly Cong Uan in downtown Hanoi to mark the 40th anniversary of the Chinese occupation of the disputed Paracels (Hoang Sa in Vietnamese) in the South China Sea, on 19 January 2014
Vietnamese protesters mark China's seizure of the Paracels in 1974
Recent flashpoints
The most serious trouble in recent decades has flared between Vietnam and China, and there have also been stand-offs between the Philippines and China:
In 1974 the Chinese seized the Paracels from Vietnam, killing more than 70 Vietnamese troops.
In 1988 the two sides clashed in the Spratlys, with Vietnam again coming off worse, losing about 60 sailors.
In early 2012, China and the Philippines engaged in a lengthy maritime stand-off, accusing each other of intrusions in the Scarborough Shoal.
In July 2012 China angered Vietnam and the Philippines when it formally created Sansha city, an administrative body with its headquarters in the Paracels which it says oversees Chinese territory in the South China Sea.
Unverified claims that the Chinese navy sabotaged two Vietnamese exploration operations in late 2012 led to large anti-China protests on Vietnam's streets.
In January 2013, Manila said it was taking China to a UN tribunal under the auspices of the UN Convention on the Laws of the Sea, to challenge its claims.
In May 2014, the introduction by China of a drilling rig into waters near the Paracel Islands led to multiple collisions between Vietnamese and Chinese ships.
In April 2015, satellite images showed China building an airstrip on reclaimed land in the Spratlys.
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In this photo taken on 29 March 2014, an aerial view shows a Philippine navy vessel that has been grounded since 1999 to assert the nation's sovereignty over the Second Thomas Shoal, a remote South China Sea reef also claimed by China
The Philippines has a rusting vessel beached on the Second Thomas Shoal, which China also claims
What does the rest of the world say?
Although China has tended to favour bilateral negotiations behind closed doors, other countries want international mediation. But even if the Philippines is successful in its attempts to pursue China at a UN tribunal, China would not be obliged to abide by the ruling.
Recent attempts by regional grouping Asean to discuss new ideas for resolving the dispute appear to have left the bloc severely divided.
The US has warned China not to "elbow aside" the countries it is in conflict with over the islands.
Q&A: South China Sea dispute - BBC News
