Earlier, former Obama adviser Ben Rhodes strongly denied Trump's allegations.

"No president can order a wiretap. Those restrictions were put in place to protect citizens from people like you," Rhodes wrote on Twitter.

Trump's administration has come under pressure from Federal Bureau of Investigation and congressional investigations into contacts between some members of his campaign team and Russian officials during his campaign.

Senator Lindsey Graham, a South Carolina Republican, said he had no knowledge about any wiretapping but is "very worried that our president is suggesting that the former president has done something illegally. I would (also) be very worried if in fact the Obama administration was able to obtain a warrant lawfully about Trump campaign activity."

Graham said it was his job "to get to the bottom of this. I promise I will."

Several other Republicans again urged an investigation into a series of intelligence-related leaks.

House Democratic Leader Nancy Pelosi ridiculed Trump's assertions. "The Deflector-in-Chief is at it again. An investigation by an independent commission is the only answer," she wrote on Twitter on Saturday.

Obama imposed sanctions on Russia and ordered Russian diplomats to leave the United States in December over the country's involvement in hacking political parties in the Nov. 8 U.S. presidential election.