China has nearly finished building structures on artificial islands in the South China Sea that could house missiles, in what may be an early political test of US President Donald Trump.

Some 20 structures have been built, capable of housing long-range surface-to-air missiles.

That would expand China's air defence umbrella over the islands. 

The United States calls this island building illegal. 

US Secretary of State Rex Tillerson angered China last month after he said it should be stopped from building on the South China Sea islands.

He compared Beijing's island claims to Russia taking Crimea from Ukraine.

Since then both Tillerson and Trump have walked back their language with Trump even pledging to honour the US "One China" policy this month in a phone call with Chinese President Xi Jinping.

Officials said the new structures don't pose a significant threat to American forces in the region- given how visible and vulnerable they are.

They say instead, it appears to be a political test of the waters to see how far Trump will go.

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