The border wall between the US and its southern neighbour is a step closer after four companies were awarded contracts to build prototypes.

One of the central promises of Donald Trump's presidential campaign was to build a wall between his country and Mexico, vowing it would stop illegal immigration.

The companies chosen will build reinforced concrete models up to 30ft (9m) high and about 30ft wide, which will be tested in San Diego.

Among the requirements for the prototypes is an "anti climb" feature.

The four companies are Caddell Construction Co of Montgomery, Alabama; Fisher Sand & Gravel Co of Tempe, Arizona; Texas Sterling Construction Co of Houston; and WG Yates & Sons Construction Company of Philadelphia, Mississippi.


Image: US President Donald Trump has promised the wall to fight illegal immigration

Contracts for non-concrete prototypes will be awarded later.

The contracts range in price from about $400,000 (£309,000) to about $500,000 but the full project could become a $20bn job spanning the 2,000-mile frontier.

Mr Trump initially vowed to make Mexico pay for the wall but Mexico refused.

Mr Trump is now trying to get Congress to fund the wall, vowing to shut the government if he does not get his way before the fiscal year begins in October.