US President Donald Trump has formally withdrawn the United States from the Trans-Pacific Partnership (TPP).

In one of his first acts in office, Trump signed an executive order on Monday morning (Tuesday morning NZ time) to pull out of the world's biggest free-trade deal, signed in Auckland last year.

Trump has repeatedly expressed his opposition to free-trade deals, and was also set to sign an order to renegotiate the North American Free Trade Agreement (Nafta) deal between the US, Canada and Mexico.

KEVIN LAMARQUE/REUTERS US President Donald Trump signed an executive order to withdraw from the TPP.

On Monday, Prime Minister Bill English warned that the TPP withdrawal would be bad for both New Zealand and the United States.

READ MORE:

* US scrapping TPP is bad for NZ - English

* Trump means TPP down but not out, Kiwis say

* TPP nations eye a future without the US

However, he was hopeful of keeping the deal alive with the remaining signatories, which are: Brunei, Chile, Singapore, Australia, Canada, Japan, Malaysia, Mexico, Peru and Vietnam.

REUTERS Opposition to the TPP had been a major theme of Trump's US election campaign.

Former US President Barack Obama strongly backed the TPP but it was never ratified by the Republican-controlled Congress.

WHAT IS AN EXECUTIVE ORDER?

In the United States, every president since George Washington has signed executive orders, which allow them to bypass Congress to change policies or laws.

KEVIN LAMARQUE/REUTERS Predecessor Barack Obama strongly backed the TPP but it was never ratified by Republican-controlled Congress.

Congress can't overturn executive orders, although federal courts can.

Presidents can also overturn their predecessors' executive orders, and Trump plans to do so with a number of former President Barack Obama's orders.

Previous executive orders have ranged from creating Japanese-Americans internment camps during World War II, to sending US soldiers to conflict zones, and stopping racial discrimination in federal agencies.

TRUMP ON FREE TRADE

Trump, who was sworn in as the 45th US president on Friday, targeted both the TPP and Nafta during his White House campaign.

However, Trump has signalled he's interested in a free-trade deal with the United Kingdom, once it completes the Brexit.

The president said on Sunday he planned talks soon with the leaders of Canada and Mexico to begin renegotiating Nafta.

"We will be starting negotiations having to do with Nafta," Trump said at a swearing-in ceremony for his top White House advisers. "We are going to start renegotiating on Nafta, on immigration and on security at the border."

Trump said during the campaign he wanted to secure more favourable terms for the United States in the Nafta pact.

Nafta, which took effect in 1994, and other trade deals became lightning rods for voter anger in the US industrial heartland states that swept Trump to victory.