US President Donald Trump has ordered four prototypes of his promised wall on the Mexican border.

The Customs and Border Protection agency announced it had awarded contracts, worth up to $500,000 each, to four construction firms.

It gave few details of the prototypes that had been selected apart from that they will all be made of concrete.

Once they are built officials will spend weeks testing their effectiveness using small hand tools to see how easily they can be damaged or breached.

Funding for four other prototypes of proposed non-concrete versions of the wall will be awarded next week.

In total all the prototypes will cost $3.6 million and they will be built near San Diego, California.

All four companies selected so far are based in the US, in the states of Alabama, Arizona, Texas and Mississippi.

Mr Trump made the construction of the wall a signature pledge during last year's election campaign.