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US Vice-President Mike Pence has urged special counsel Robert Mueller to "wrap up" his investigation into alleged Russian meddling in the 2016 election.

He said the White House has "fully co-operated" with the probe, which is looking at whether US President Donald Trump's campaign colluded with Russia.

Mr Trump and the Kremlin have repeatedly denied the allegations.

His remarks came as he and Mr Trump greeted three Americans returning from North Korea after being held there.

"In the interests of the country, I think it's time to wrap it up," he told NBC News early on Thursday morning at Joint Base Andrews.

The vice-president added the administration had provided more than a million documents to Mr Mueller's investigation team.

"And I would very respectfully encourage the special counsel and his team to bring their work to completion."

Mr Pence dismissed reports about Michael Cohen, Mr Trump's personal lawyer, receiving millions of dollars in payments from companies as a "private matter" and "something I don't have any knowledge about".

Earlier this week, the New York Times reported that Mr Cohen's company, Essential Consultants LLC, last year received half a million dollars from Columbus Nova, a New York-based investment company affiliated with a firm controlled by Russian oligarch Viktor Vekselberg.

It was one of several businesses - including major corporations like AT&T - which paid a shell company set up by Mr Cohen after the 2016 election.

Following the vice-president's comments, top Democrat Adam Schiff tweeted that Mr Pence "has now brought his sycophancy to a whole new level".

Mr Schiff's tweet centers around lingering concerns over Michael Flynn, the former US national security adviser, who pleaded guilty to lying to the FBI about his contacts with Russia last December.

Mr Trump sacked Mr Flynn last February, saying he had lied to Mr Pence about meeting the Russian envoy to the US.

Per a list of questions leaked to US media, it appeared that special counsel Robert Mueller will look into Mr Flynn's contacts with Russian Ambassador Sergei Kislyak and what exactly the president knew about the situation.