SAN DIEGO - The US Customs and Border Protection has released pictures showing the construction of several prototypes for the controversial US-Mexico border wall in San Diego County.

Eight nine-metre tall prototypes, four made of concrete and four of other materials, will be erected.

Media reports said that work on the prototypes started one week ago and all eight mock-ups will be completed in a month.

Following the completion of the prototype, each model will undergo testing to measure resistance to tampering and penetration.

President Donald Trump says he will pick the winning design in a month even though no funding has been approved by Congress yet for a border wall which he first proposed on his campaign trail to stop illegal crossing.

During his 2016 presidential campaign, Mr Trump promised an entirely concrete wall modelled on those in Israel.

During a Senate hearing in April, then Secretary of Homeland Security John Kelly said it is "unlikely that we will build a wall or physical barrier from sea to shining sea" like Trump suggested during his campaign.

Kelly said he believed significant sections of the border would consist of only electronic monitoring.

The mock-ups are being built near the Otay Mesa Port of Entry in San Diego by several companies that pitched designs to the Department of Homeland Security (DHS) in March.

The prototypes "are taking form or are near completion," CBP's San Diego branch tweeted.

An analysis by DHS in February revealed by Reuters found Trump's border wall proposal could cost as much as US$21.6 billion to build.

During the election campaign Trump promised voters the wall would cost roughly US$12 billion, but he originally said Mexico would pay for it.

Mexico's president Enrique Peña Nieto has repeated that Mexico would not put a single peso towards the US president's signature project.