Russian President Vladimir Putin mocked US special counsel Robert Mueller's investigation Tuesday, saying 'a mountain gave birth to a mouse' and the probe validated the Kremlin's continuous denials of collusion with President Donald Trump's campaign.

In his first comments since Mueller presented his investigation report last month, Putin said claims of a campaign conspiracy of Democrats reflected their failure to accept the outcome of the 2016 presidential election.

'It was clear for us from the start that it would end like this,' the Russian leader said of the Trump-Russia investigation. 'A mountain gave birth to a mouse.'

Attorney General William Barr wrote in a summary of Mueller's still-confidential report that the special counsel found no evidence the Trump campaign 'conspired or coordinated' with the Russian government to influence the election.

However, Mueller's long investigation uncovered a multi-pronged operation to interfere with the 2016 vote and indicted 25 Russians on charges of hacking Democratic email accounts and spreading disinformation on social media.

Putin on Tuesday repeated Moscow's across-the-board disavowals, including that the Russian government had no contact with Trump when he visited Moscow as a businessman.

'We have been saying from the start that this notorious commission led by Mr Mueller won't find anything, because no one knows better than us: Russia has not meddled in any US election,' Putin said at an Arctic forum in St. Petersburg. 'There was no collusion between Trump and Russia that Mr Mueller was looking for.'

The Russian leader described the claims of collusion between Trump and Russia as 'sheer nonsense aimed at a domestic audience and used for domestic political infighting in the United States.'

Putin said he thinks political tensions in the US haven't abated since Mueller finished his investigation alleged Trump's foes are 'searching for new pretexts' to attack the president.

'Look what is going on now. Those groups which are attacking the legitimately elected president, what are they doing?' Putin said. 'They disagree with the choice of the American people and seek to reduce the results to zero. This is a crisis of a political system, which we have never seen in US history.'

He cast the continued pressure on Trump as disrespect to voters and charged that top Democrats 'place their party interests above national interests.'

'I'm not defending President Trump, we have plenty of disagreements,' Putin went on. 'His administration has introduced numerous sanctions against Russia, something we disagree with and will never accept. We consider it counterproductive.'

Putin said he hopes partisan fighting in Washington eases so the US and Russia can focus on issues of mutual interest, such as nuclear arms control, fighting terrorism and climate change.

'If Russia and the United States resume a full-fledged equal dialogue on issues of interest to us and the entire humankind, including arms control, we will be happy to see that,' he said.

Asked if Russia would support Trump's re-election in 2020, Putin said that Moscow would 'respect the choice of the American people.'