According to Reuters, the report has not yet been presented to DHS Secretary John Kelly.

Kelly also said during the interview that he hoped the project would be completed within two years - significantly quicker than the DHS report estimates.

With 654 miles (1,046 km) of the border already fortified, the new construction would extend nearly the length of the entire border. "I am reading that the great border WALL will cost more than the government originally thought, but I have not gotten involved in the. design or negotiations yet", Mr Trump tweeted from his Florida resort, where he is hosting Japanese Prime Minister Shinzo Abe. Neither the White House nor the Department of Homeland Security have confirmed the report, but here's a breakdown of the numbers seen in the report obtained by Reuters.

Homeland Security reportedly is banking on having money from Congress for a border barrier by April or May and plans to begin building it in September.

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Reuters also says the document points out that Trump will have to meet several strict requirements of the International Boundary and Water Commission, part of an international treaty between Mexico and the USA, which could cost between US$11 million and US$15 million per mile in some areas. Under this plan, the US would refrain from taxing American companies' exports, but would tax imports. It also shows the government has begun working with existing contractors and planning steel purchases for the project.

Mr Trump told law enforcement officials this week: "The wall is getting designed right now". During his presidential campaign Trump had cited a $12 billion figure.

The border, which is about 1,900 miles long, already has some fences in the area. The DHS estimate represents the cost to build another 1,250 miles by the end of 2020.

Reuters quoted a source within DHS saying that the Trump administration would likely need to seize hundreds of miles of the privately-owned borderland from US citizens under eminent domain in order to embark on the final phase. An independent estimate by a construction firm suggests that could easily run to $25 billion given uncertainties in the plan. But the rest of the construction will be markedly more expensive, covering a much larger stretch of land, much of it privately owned or inaccessible by road. While the first phase will be relatively affordable, at some $360 million, each successive addition will require the USA government to acquire the land via eminent domain as well as through environmental waivers.