It is "almost impossible" that Donald Trump would not have known about Russian hacking during the US presidential election, a former ambassador to Moscow said as Barack Obama vowed that America would respond to the action.
The outgoing president said the US would react at a "time and place of our choosing" to the hacking of Democratic officials' emails in the run-up to the election.
The UK's former ambassador to Russia, Sir Andrew Wood, said there was a "case to answer" for Mr Trump about the hack.
Mr Obama told NPR News that he had spoken directly to President Vladimir Putin about his feelings over the hacking.
Whenever a foreign government tried to interfere in US elections, the nation must take action - "and we will", Mr Obama stated.
"Some of it may be explicit and publicised, some of it may not be. But Mr Putin is well aware of my feelings about this, because I spoke to him directly about it."
On Thursday the Obama administration suggested Mr Putin personally authorised the hacking of Democratic officials' email accounts in the run-up to the election and said it was "fact" that such actions helped Mr Trump's campaign.
And the White House also attacked the president-elect himself, saying he must have known of Russia's interference.
That view was echoed by Sir Andrew, who told BBC Radio 5 Live: "I don't see how Donald Trump could not have known something. I think that's almost impossible. What he actually said to Hillary Clinton in the debates was, essentially, 'You can't prove it - you don't know'. He never said 'This is serious, it must be investigated'.
Sir Andrew, who was ambassador to Russia from 1995 to 2000, added: "He did have people around him who had quite a lot to do with Russia or Ukraine. So I think there is a case to answer."
The Kremlin has rejected Washington's suggestions that President Putin was involved in the hacks, with spokesman Dmitry Peskov dismissing it as "laughable nonsense".
Kellyanne Conway, Mr Trump's senior transition adviser, said it was "breathtaking" and irresponsible that the White House had suggested Mr Trump knew Russia was interfering to help his campaign.