China’s territorial claims in the South China Sea – through which about £3.9trillion in ship-borne trade passes each year – are contested by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.
In his speech at the annual Shangri-La Dialogue forum, General Mattis said: “We oppose countries militarising artificial islands and enforcing excessive maritime claims.
“We cannot and will not accept unilateral, coercive changes to the status quo.”
The South China Sea row has reached boiling point
During his nomination hearing earlier this year, Secretary of State Rex Tillerson warned that the US was “going to have to send China a clear signal that first the island-building stops, and second your access to those islands also is not going to be allowed.”
In response, the Chinese foreign ministry said Beijing would “remain firm to defend its rights in the region”.
Itu Aba Island in the South China Sea
Its islets and waters are claimed in part or in whole by Taiwan, China, Vietnam, the Philippines, Malaysia and Brunei.
As well as this, Beijing has been building artificial islands on reefs and carrying out naval patrols in waters also claimed by these other nations.
Although the previous US administration of Barack Obama insisted it was neutral territory, it spoke out strongly against the island-building and sought to build ties with, and among, the South East Asian nations whose claims overlap those of China.
Woody Island is the largest of the Paracel Islands in the South China Sea
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, showing the two island groups falling entirely within its territory.
However, critics say China has not clarified its claims sufficiently – adding that the nine-dash line that appears on Chinese maps encompassing almost the entirety of the South China Sea includes no coordinates.
It is also not clear whether China claims only land territory within the nine-dash line, or all the territorial waters within the line as well.