March 5, 2017 - 4:41pm
US President Donald Trump has been urged to provide evidence to back his allegation that his predecessor, Barack Obama, ordered his phones to be tapped during the election campaign.
Republican Senator Ben Sasse said Mr Trump's comments were "serious" and he should explain the alleged wire-tapping and how he came to know about it.
The Republican president has supplied no details to back his claim.
An Obama spokesman said he had never ordered surveillance of any US citizen.
Mr Trump's tweets follow allegations made by conservative radio host Mark Levin, including that the Obama administration "sought, and eventually obtained, authorisation to eavesdrop" on the Trump campaign last year.
Other media reports had previously suggested the FBI had sought a warrant from the foreign intelligence surveillance court (Fisa) in order to monitor members of the Trump team suspected of irregular contacts with Russian officials.
The warrant was first turned down but then reportedly approved in October, though there has been no official confirmation.
Under Fisa, wire-tapping can only be approved if there is probable cause to believe that the target of the surveillance is an agent of a foreign power. Mr Obama could not lawfully have ordered such a warrant.
Mr Trump, who has been facing intense scrutiny over alleged Russian interference in support of his election campaign, made the allegation in a series of tweets on Saturday.
Writing from his weekend home in Florida, he called the alleged tapping "Nixon/Watergate", referring to the most notorious political scandal of 1972, which led to the downfall of President Richard Nixon after a web of political spying, sabotage and bribery was exposed by the media.
The spokesman for Mr Obama, Kevin Lewis, said the accusation was "simply false".
A "cardinal rule of the Obama Administration was that no White House official ever interfered with any independent investigation led by the Department of Justice", he said.
The statement left open the possibility that a judicial investigation had been taking place.
Earlier Ben Rhodes, who was Mr Obama's foreign policy adviser and speechwriter, also addressed Mr Trump's claims in a tweet, saying: "No President can order a wire-tap. Those restrictions were put in place to protect citizens from people like you."