President-elect Donald Trump said for the first time he believes Russia was behind hacking ahead of the election.

"I think it was Russia," Trump said during his first news conference as President-elect.

He added that Russia is not the only nation that hacks US targets and accused Democrats of not having sufficient cybersecurity programs.

The news conference opened with his spokesman, Sean Spicer, slamming a "political witch hunt" following reports that Russian operatives claim to have compromising personal and financial information about Trump.

Vice President-elect Mike Pence also criticized the media before introducing Trump, who kept up his criticism of US intelligence.

"I do have to say and I must say that I want to thank a lot of the news organizations here today because they looked at that nonsense that was released by maybe the intelligence agencies," Trump said. "But maybe the intelligence agencies which would be a tremendous blot on their record if they did that. A thing like that should never have been written, it should never have been had and it certainly should have never been released."

He reiterated that he doesn't plan to release his tax returns, saying they are under audit and don't include relevant information

The news conference follows exclusive reporting by CNN on Tuesday that classified documents presented last week to President Barack Obama and Trump included the allegations about Russia. The allegations were presented in a two-page synopsis that was appended to a report on Russian interference in the 2016 election and drew in part from memos compiled by a former British intelligence operative, whose past work US intelligence officials consider credible.

The FBI is investigating the credibility and accuracy of these allegations, which are based primarily on information from Russian sources, but has not confirmed many essential details in the memos about Mr. Trump.