Donald Trump’s former campaign manager Paul Manafort held secret talks with Julian Assange inside the Ecuadorian embassy in London, and visited around the time he joined Trump’s campaign, the Guardian has been told.

Sources have said Manafort went to see Assange in 2013, 2015 and in spring 2016 – during the period when he was made a key figure in Trump’s push for the White House.

It is unclear why Manafort would have wanted to see Assange and what was discussed. But the last apparent meeting is likely to come under scrutiny and could interest Robert Mueller, the special prosecutor who is investigating alleged collusion between the Trump campaign and Russia.

A well-placed source has told the Guardian that Manafort went to see Assange around March 2016. Months later WikiLeaks released a stash of Democratic emails stolen by Russian intelligence officers.