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.