Image copyright Getty Images Image caption Donald Trump has cast doubt on the findings of the intelligence agencies co-ordinated by Gen James Clapper (right)

Hacking had no effect on the outcome of the US election, President-elect Donald Trump has said after a meeting with intelligence chiefs.

However, Mr Trump said he would ask for a plan within 90 days of taking office on how to stop cyber-attacks.

US intelligence chiefs believe Russia was behind the hacking of Democratic Party emails to damage Hillary Clinton.

But hours before the briefing, Mr Trump dismissed the claims as a "political witch-hunt" by his opponents.

He told the New York Times that US institutions had been the target of previous hacking attacks, but they had not received the same media attention as the alleged intrusions during the election campaign.

Later, Mr Trump described the briefing at Trump Tower by National Intelligence Director Gen James Clapper, CIA Director John Brennan and FBI Director James Comey as "constructive".

In a statement he declined to single out Russia and said he had "tremendous respect for the work and service done" by those in the US intelligence community.

"While Russia, China, other countries, outside groups and people are consistently trying to break through the cyber infrastructure of our governmental institutions, businesses and organisations including the Democrat National Committee, there was absolutely no effect on the outcome of the election."

He added: "Whether it is our government, organisations, associations or businesses we need to aggressively combat and stop cyber-attacks. I will appoint a team to give me a plan within 90 days of taking office."

Since winning the election, Mr Trump has repeatedly questioned intelligence on the hacking claims.

And in a tweet on Friday, Mr Trump said he wanted an investigation into how "top secret intelligence" was shared with US broadcaster NBC "prior to me seeing it".

An unclassified version of the intelligence report will be made public next week, but some details have already started emerging in US media, including on NBC.

The identities of the Russian agents behind the hack are known to US authorities but have not been released, reports say, citing intelligence sources.

Earlier on Friday, Sean Spicer, who will become White House communications director under Mr Trump, told ABC's Good Morning on Friday that the president-elect would go into the meeting "prepared to listen and understand how they got to the conclusions''.

Image copyright AP Image caption Vice-president elect Mike Pence arrived at Trump Tower ahead of the briefing

The Russian agents are alleged to have sent stolen Democratic emails to WikiLeaks to try to swing the vote for Mr Trump.

Russia has denied any involvement in the hacking and WikiLeaks founder Julian Assange says Moscow was not the source for the site's mass leak of emails from the Democratic Party.

According to CNN, the Washington Post and NBC News, citing intelligence sources, agencies had intercepted communications following the election which showed senior Russian government officials celebrated Donald Trump's win over rival Hillary Clinton.

"The Russians felt pretty good about what happened and they also felt pretty good about what they did," a senior US official quoted by the Washington Post said.

Image copyright AFP Image caption Preparations for Donald Trump's inauguration on 20 January are under way in Washington

NBC says the alleged Russian hacking targeted not just the Democratic National Committee (DNC) but also the White House, joint chiefs of staff, the department of state and large US corporations.

Mr Trump has repeatedly rejected allegations that the Russian government was behind the hacks.

On Wednesday, he repeated Julian Assange's claim that "a 14-year-old" could have carried out the breach, and said the DNC was "careless".

On Thursday, he said he was a "big fan" of intelligence agencies, but appeared to question their findings in a tweet, asking why they decided not to request to examine computers belonging to the DNC.

Outgoing Vice-President Joe Biden lambasted the president-elect on Thursday for attacking the intelligence community over the claims, saying it was "absolutely mindless" not to have faith in the agencies.

Media playback is unsupported on your device Media caption Joe Biden to Donald Trump: "Grow up, time to be an adult, you're president"

He told the PBS network that he had read a US intelligence report that clearly confirmed Russia had tried to "discredit the US electoral process" as part of a systematic campaign to undermine Mrs Clinton.

Her campaign manager, John Podesta, was among those whose emails were hacked, as well as the DNC.

Mr Biden also criticised Mr Trump for ignoring intelligence on the hack.

"The idea that you may know more than the intelligence community knows - it's like saying I know more about physics than my professor. I didn't read the book, I just know I know more," he said.

On Thursday, Gen Clapper told a hearing of the Senate Armed Services Committee that Russian President Vladimir Putin had ordered the hack, and said the motive would be revealed next week.

Media playback is unsupported on your device Media caption Obama's former director of cyber security warns Russia hacking could be used as "playbook"

Republican Senator John McCain, a leading Russia critic in Congress who chaired the hearing, said it was in the interests of all Americans to confront the problem of foreign hacking.

"There is no national security interest more vital to the US than the ability to hold free and fair elections without foreign interference."

Last week President Barack Obama ordered the expulsion of 35 Russian diplomats from the US over the alleged hacking. Russia has said it will not reciprocate.