The Trump administration has proposed spending $18 billion over 10 years to significantly extend the border wall with Mexico.

The proposal provides one of the most detailed blueprints of how the president hopes to carry out a signature campaign pledge.

A U.S. official with direct knowledge of the matter says Customs and Border Protection is calling for 316 miles (505 kilometers) of additional barrier by September 2027, bringing total coverage to nearly half the border.

The official spoke to The Associated Press on condition of anonymity because the plan has not been made public.

The proposal, first reported by The Wall Street Journal, also calls for 407 miles (651 kilometers) of replacement or secondary fencing.