Assange's claims run contrary to the Senate intelligence committee's findings that Russian Federation was behind the hacking of the DNC.
The WikiLeaks legal team said they are "considering the offer but testimony must conform to a high ethical standard".
Despite Assange's central role in the release of the DNC emails, the Senate intelligence committee's testimony request appears to the first time that investigators reached out to the Wikileaks founder. The letter says the committee requests Assange make himself available for a closed-door interview "at a mutually agreeable time and location".
On Wednesday, WikiLeaks said the Senate letter - dated August 1 - was delivered to Assange via the United States embassy in London, but the mission declined to comment when contacted by AFP.
Britain has said it will arrest Assange if he leaves the embassy as he skipped bail in order to take refuge in the building.
Moreno told the Spanish newspaper El Pais in an interview in the last week of July that Ecuador was not helping Assange to exercise his rights by prolonging his stay in Ecuador's London embassy, and therefore had to find a way to end the impasse.
Julian Assange, who has been holed up in the Knightsbridge residence since 2012, over fears he will be extradited to the United States to face prosecution over WikiLeaks' publication of leaked USA military and diplomatic documents.
"If the British government guarantees us that he will not run the risk of being extradited to another country, we will ask Mr. Assange (to leave); we will talk first with his attorneys", Moreno said.
Ahead of the 2016 election, WikiLeaks published emails hacked from the Democratic Party and the personal email account of John Podesta, Hillary Clinton's campaign chairman.