Trump calls Syrian refugees 'illegals' in NATO-bashing, Brexit-backing interview

US President-elect Donald Trump called Syrian refugees seeking safety in Germany "illegals" as he slammed Chancellor Angela Merkel's open door policy as a "catastrophic mistake".

The incoming president also called NATO "obsolete" and hailed Britain's exit from the European Union in an interview published in Monday editions of The Times of London and Germany's Bild.

He predicted more countries would leave the EU, largely due to the pressure the bloc was put under following a significant uptick in refugees arriving.

"If they hadn't been forced to take in all of the refugees, so many, with all the problems that it... entails, I think that you wouldn't have a Brexit. This was the final straw that broke the camel's back," he said.

In criticising Merkel for letting Germany admit undocumented refugees enter the country, he insinuated that this posed a security risk.

"I think she made one very catastrophic mistake and that was taking all of these illegals, you know taking all of the people from wherever they come from. And nobody even knows where they come from," Trump said.

In other remarks, Trump said Brexit "is going to end up as a great thing" and said he backed a trade deal with post-EU Britain, which would be "good for both sides."

"We're gonna work very hard to get it done quickly and done properly," said Trump, confirming he will meet British Prime Minister Theresa May soon after his inauguration on 20 January.

"NATO obsolete"

In remarks likely to rattle US allies, Trump reiterated his campaign trail comments that he would think twice about helping NATO allies if the United States were not "reasonably reimbursed" for the costs of defending them.

"I said a long time ago that NATO had problems.

"Number one, it was obsolete, because it was designed many, many years ago," he said.

"Number two, the countries aren't paying what they're supposed to pay.

"I took such heat, when I said NATO was obsolete. It's obsolete because it wasn't taking care of terror. I took a lot of heat for two days. And then they started saying Trump is right."

Spending has been a common source of friction within NATO over recent years. 

The core military contributor to the alliance is the US, which accounts for about 70 percent of spending. 

"The countries aren't paying their fair share so we're supposed to protect countries," Trump said in Sunday's interview.

"(...) There's five countries that are paying what they're supposed to. Five. It's not much."

NATO members pledge to defend each other if attacked, although the only time the self-defence clause has been invoked was after the 9/11 terror attacks on the US.