American President Donald Trump is to inspect prototypes for a border wall which would eventually create a giant physical barrier between the US and its southern neighbor, Mexico. His visit to California comes amid an intensifying feud between the White House and the country's most populous state over immigration polices. The prototypes -- made of concrete, steel and other materials -- have been built near the city of San Diego. A winning design has yet to be chosen amid uncertainty about funding and the wall's overall plan.