The Trump administration has proposed spending $US18 billion over 10 years to significantly extend the border wall with Mexico.
The administration has now provided one of its most detailed blueprints of how the president hopes to carry out a signature campaign pledge.
The proposal by Customs and Border Protection calls for 505 kilometres of additional barrier by September 2027.
This would bring total coverage to 1,552 kilometres, or nearly half the border.
Mexico has steadfastly rejected President Trump's demand that Mexico pay for the wall and few doubt that US taxpayers will foot the bill - if the wall ends up being built.
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