The United States will start awarding contracts within weeks for a proposed border wall with Mexico, with President Donald Trump declaring it to be “way, way, way ahead of schedule”.

US Customs and Border Protection announced on Saturday (AEDT) it would request bids for the project on or around March 6 with companies required to submit concept designs and prototypes by March 10, according to a website for federal contractors.

Finalists must submit offers with their proposed costs by March 24.

Mr Trump told the Conservative Political Action Conference (CPAC) that construction would start “very soon” and was “way, way, way ahead of schedule”.

But Mexico has hit back, saying it would retaliate if the US imposed a border tax and that it could afford to lose financial aid that might be pulled to pay for the wall.

Mexico’s Foreign Minister Luis Videgaray said on Saturday (AEDT) the nation could respond to any tax the US were to impose on imports from Mexico to finance the wall with levies on goods.

“Without a doubt, we have that possibility, and what we cannot do is remain with our arms crossed,” Mr Videgaray said in a radio interview.

The statement signalled a distinctly defiant tone from Mexico since President Enrique Pena Nieto last month cancelled a trip to meet Mr Trump over the wall dispute.

Meanwhile, it is still unclear how soon Congress would provide funding and how much.

The Government Accountability Office estimates it will cost on average $US6.5 million ($A8.5 million) per mile for a fence and $US1.8 million ($2.35 million) a mile for vehicle barriers.

There are currently 586km of pedestrian fencing and 500km of vehicle barriers, much of it built during President George W. Bush’s second term.

Republican leaders in Congress have said Mr Trump’s wall would cost between $US12 billion ($A15.6 billion) and $US15 billion ($A19.6 billion). Mr Trump has suggested $US12 billion.

Mexico City performance fells Trump 'wall' amid cheers https://t.co/66bBVwGKMK pic.twitter.com/MqZbbVTrvb

— Reuters TV (@ReutersTV) February 21, 2017

An internal Homeland Security Department report estimates the cost of extending the wall along the entire US-Mexico border at about $US21 billion ($A27.3 billion), according to a US government official who is involved in border issues.

At the same CPAC conference, Mr Trump also vowed a bigger, better travel ban, attacked the media and pledged a “massive” budget request to build up the army to achieve “one of the greatest military build-ups” in American history.

“Hopefully, we’ll never have to use it, but nobody is going to mess with us. Nobody,” Mr Trump said.

White House bars media from press briefing

Mr Trump barred major media outlets such as The New York Times, the Los Angeles Times, CNN and Politico from a White House press briefing on Friday (US time).

Typically, the daily briefing is televised and open to all news organisations credentialed to cover the White House.

Only hours after Mr Trump gave a speech blasting the media, White House press secretary Sean Spicer invited only a pool of news organisations that represented and shared reporting with the larger press corps.

FAKE NEWS media knowingly doesn't tell the truth. A great danger to our country. The failing @nytimes has become a joke. Likewise @CNN. Sad!

— Donald J. Trump (@realDonaldTrump) February 25, 2017

He also invited several other major news outlets, as well as smaller organisations, including the conservative website Breitbart News, whose former executive chairman, Steve Bannon, is Mr Trump’s chief strategist.

When additional news organisations including The New York Times, the Los Angeles Times, CNN and Politico attempted to gain access, they were not allowed to enter.

Former George W Bush spokesman Ari Fleischer defended Mr Spicer’s actions on Twitter while a Politico video of Mr Spicer in December saying “barring media access is what a ‘dictatorship’ does,” became widely circulated on social media.

Davan Maharaj, editor-in-chief and publisher of the Los Angeles Times, called the newspaper’s exclusion “unfortunate”.

“The public has a right to know, and that means being informed by a variety of news sources, not just those filtered by the White House press office in hopes of getting friendly coverage,” Mr Maharaj said in a statement.

Regardless of access, the Times will continue to report on the Trump administration without fear or favour.”

Dean Baquet, executive editor of The New York Times, said: “Nothing like this has ever happened at the White House in our long history of covering multiple administrations of different parties.

”We strongly protest the exclusion of The New York Times and the other news organisations.

”Free media access to a transparent government is obviously of crucial national interest,” he said.

– with AP/ABC