In a statement, WikiLeaks said the questioning was scheduled for 18 January US time (which would be approximately 19 January Australian time), adding that it had been made in 7 January.
It said the request had been made to the Ecuadorian authorities who had approved it, despite it being "highly unusual to permit foreign interrogations of former diplomatic officials over their diplomatic work, or to provide foreign investigators information about those who have been afforded political asylum in relation to them".
The Guardian story, which ran on 28 November, was denied by Assange, Manafort and the former consul of Ecuador at the embassy.
The Washington Post ran a critical story about the claims and The Intercept ran a couple of yarns, questioning why there was no security footage of Manafort entering or leaving the embassy despite the fact the area surrounding the embassy has a large number of security cameras installed.
Additionally, questions were raised as to why none of the British policemen, who have been stationed outside the embassy since Assange took refuge there in June 2012, was able to confirm Manafort's alleged visits.
The subpoenas were issued after Democrat Senators Dianne Feinstein, Chuck Schumer and Bob Menendez wrote to US Secretary of State Mike Pompeo demanding that Washington ask Ecuador to provide it with information about the allegations in the story.
Assange has said that he fears being extradited to the US if he leaves the embassy in London. Last year, US officials said arresting him was a top priority.
In November 2018, it emerged that the US may have already filed charges against Assange. This came to light after portions of a complaint against him were apparently cut and pasted into a complaint against an unrelated individual, Seitu Sulayman Kokayi.
Assange's problems began when he visited Sweden in August 2010 to attend a conference where he was scheduled to give a talk. During that visit, he had sex with two women whom he met. The pair filed rape and molestation complaints against him later, claims that he denied.
He was questioned by Swedish authorities and cleared. He could have left the country then and there, but stayed for a while, in case the authorities decided to question him again.
Interpol issued a Red Notice for his arrest on 20 November 2010. On 27 November, Assange surrendered to authorities and appeared before a Westminster judge. He was granted bail in December after his backers provided £240,000 in cash and sureties.
A legal back and forth eventuated and went on until June 2012, when Swedish prosecutors sought his extradition.
Assange's lawyers, among them the world-renowned Australian Geoffrey Robertson, replied that if he agreed to the extradition request, then he could be flown to the US from there.
On 19 June 2012, he jumped bail and took refuge in the Ecuadorian embassy, seeking asylum in the South American country. British police surrounded the building and blocked any chance of his leaving.
Ecuador granted him asylum in August 2012. He has had to stay inside the four walls of the embassy since then. He was granted Ecuadorian citizenship by a former president of the country, Rafael Correa.