White House press secretary Sean Spicer appeared to soften Donald Trump's unproven assertion Barack Obama "wire tapped" phones at Mr Trump's New York skyscraper, saying the phrase — as it appeared in an early morning Twitter spray — was not meant to be taken literally.

Key points: John McCain calls for proof or a retraction from the President

John McCain calls for proof or a retraction from the President Department of Justice requests more time to investigate

Department of Justice requests more time to investigate Kellyanne Conway says microwaves can be used for spying

More than a week after Mr Trump levelled his allegations, Mr Spicer noted the President had put the term in quotation marks.

"The President used the word wiretap in quotes to mean broadly surveillance and other activities," Mr Spicer said.

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He also suggested Mr Trump was not accusing the former president specifically, but instead referring to the actions of the Obama administration.

Current and former administration officials have been unable to provide any evidence of the Obama administration wiretapping Trump Tower, yet the President's aides have been reluctant to publicly contradict their boss.

Mr Trump's critics have slammed the President for making the wiretapping claim on his Twitter account without evidence.

Wiretapping a US citizen would require special permission from a court, and Mr Trump as president would have the ability to declassify that information.

Senator John McCain said:

I think the president has one of two choices: either retract or provide the information that the American people deserve.

"If his predecessor violated the law … we have got a serious issue here, to say the least," Mr McCain said.

The Department of Justice was due to deliver proof of the allegation by Monday (local time) but have requested extra time to do so.

The department requested "additional time to review the request in compliance with the governing legal authorities and to determine what if any responsive documents may exist," a spokeswoman said in a statement.

Senior White House advisor Kellyanne Conway said that, despite the lack of proof to support the wire-tapping claims, computers, mobile phones and even microwaves could be used for surveillance.

Ms Conway appeared to point towards the recent WikiLeaks release of nearly 8,000 documents that purportedly reveal secrets about the CIA's tools for breaking into targeted computers, mobile phones and even smart televisions.

"What I can say is there are many ways to surveil each other now, unfortunately … [including] microwaves that turn into cameras, et cetera," Ms Conway said.

AP