US President Donald Trump has signed an executive order designed to start fulfilling his campaign pledge to build a wall along the US border with Mexico. The directives ordered the construction of a multibillion-dollar wall along the roughly 3,200-km US-Mexico border , moved to strip federal funding from “sanctuary” states and cities that harbour illegal immigrants. Trump’s plans prompted an outcry from immigrant advocates and Democratic lawmakers who said the president was jeopardising the rights and freedoms of millions of people while treating Mexico as an enemy, not an ally. The immigration crackdown has sparked fear among so-called “dreamers,” whose parents brought them to the US illegally and who received temporary deportation relief and work permits from former President Barack Obama. The plan has infuriated Mexicans and President Enrique Pena Nieto said that he “regrets and disapproves” of the push by Trump to build a new wall along the border. The cost, nature and extent of the wall remain unclear. Trump also ordered a survey of the border to be completed within 180 days. He promised to make Mexico pay for the wall, something the Mexican government has repeatedly said it will not do. Trump cannot just do as he pleases with the land along the border because much of it is privately owned. So building a wall would entail messy legal proceedings and substantial expropriation payments. The Rio Grande River forms the natural border between Mexico and Texas. Laws prohibit construction that would impede flood management. And a treaty bars either country from diverting any flow of water. Estimates prepared a decade ago put costs at $6.5 million a kilometre for pedestrian fencing and $1.8 million per kilometre for vehicular blockades. An actual wall constructed of concrete or brick would be more costly and difficult. Critics say the wall proposal is pointless because, among other things, most drugs smuggled into the US pass through legal entry points and are not transported through the desert.