An article has appeared in the New York Post focused on an email from April 2015, in which an adviser to Burisma apparently thanked Hunter Biden for inviting him to meet his father in Washington.

Hunter, Joe Biden's second son, was a director on the board of Burisma - a Ukrainian-owned private energy company while his father was the Obama administration's pointman on US-Ukrainian relations. Hunter was one of several foreigners on its board.

What does the Biden say?

The New York Post article did not provide evidence that the meeting ever took place. The Biden election campaign said there was no record of any such meeting on the former vice-president's "official schedule" from the time.

But in a statement to Politico, the campaign also acknowledged that Mr Biden could have had an "informal interaction" with the Burisma adviser that did not appear on his official schedule, though it said any such encounter would have been "cursory".