The border wall proposed to be built along the US-Mexico border by President Donald Trump would be a series of walls and fences that could cost about $21 billion, and take more than three years to build, according to a US government estimate.

A US Department of Homeland Security internal report seen by Reuters on Thursday shows that the estimated price for the wall is much higher than the $12 billion figure cited by Trump during his presidential election campaign.

The report says the border wall would be built in three phases covering just over 1,250 miles (2,000 km) by the end of 2020.

The report estimates that the first phase would cost just $360 million, a relatively easy way for Trump to make good on his campaign pledge to limit illegal migration.

But the rest of the construction will be significantly more expensive, covering a much larger stretch of land, much of it privately owned or consisting of major physical barriers, like mountains and rivers.

Many cost estimates and timelines have been offered since Trump campaigned on the promise of building a wall.

 * US President Donald Trump signs an executive order to start the Mexico border wall project at the Department of Homeland Security in Washington, DC, on January 25, 2017. (AFP photo)

Building a wall on the US-Mexico border to stop the influx of refugees and undocumented migrants coming from Central and Latin America was a hallmark of Trump's presidential campaign.

During his campaign, Trump also characterized Mexican migrants living in the US as murderers and rapists and pledged to build a wall that he said Mexico would pay for.

Mexican President Enrique Peña Nieto cancelled a scheduled meeting with Trump at the White House on January 26 after the new American leader pressured him to fund construction of the border wall.