WASHINGTON (AFP) -- President Donald Trump on Wednesday ordered work to begin on planning and building a wall on the Mexican border, sounding a hardline tone on immigration as he moved to fulfill a key campaign pledge.

The US leader instructed officials to begin to "plan, design and construct a physical wall along the southern border" and -- perhaps more problematically -- see how it could be funded.

"A nation without borders is not a nation," Trump said, echoing former president Ronald Reagan, as he visited the Department of Homeland Security to sign two executive orders.

"Beginning today, the United States of America gets back control of its borders," the Republican president said.

Stemming immigration was a central plank of Trump's election campaign.

His signature policy prescription was to build a wall across the 2,000-mile (3,200-kilometer) border between the United States and Mexico.

Some of the border is already fenced, but Trump says a wall is needed to stop illegal immigrants entering from Latin America.

The policy has become a clarion call for the US right and far-right -- the core of Trump's support.

A Morning Consult/Politico poll released Wednesday said 47 percent of voters support building a wall, with 45 percent against.

Experts have voiced doubts about whether a wall would actually stem illegal immigration, or if it is worth the billions it is expected to cost.

"I suspect that a lot of Trump supporters would be just as happy with a big statue of a middle finger pointed south," said Congressman Luis Gutierrez.

"Both are about equally effective as national security strategies."

Despite the high octane rhetoric, Trump's action was piecemeal, looking to identify existing funds that could be diverted toward the project.

The Republican-controlled Congress would need to supply billions of dollars more if the wall is to be anywhere near completed.

Trump's party has spent the last decade preaching fiscal prudence, so cuts to existing programs would likely be required.

Trump also ordered a survey of the border to be completed within 180 days.

Much of the land needed to build the wall would have to be seized from private citizens in Texas, the state of Texas or tribal authorities.

That could lead to lengthy legal proceedings, political blowback and substantial expropriation payments.

"The only real solution to reform our immigration system is to pass comprehensive immigration reform that provides a path to citizenship for the 11 million" undocumented people in the United States, top Senate Democrat Chuck Schumer said.