According to the news outlet's source that chose to remain anonymous, the request was sent on Thursday.
According to a US-UK treaty, any extradition request must be filed within 60 days of an arrest and Assange was arrested on April 11 at the Ecuadorian Embassy in London.
By that same treaty, the United States can only prosecute Assange based on the charges outlined in the extradition request, which is an 18-count indictment that was filed last month on charges of Assange violating the US Espionage Act and conspiring to hack a government computer.
READ MORE: Ex-Ecuadoran Foreign Minister Believes Assange to Be Extradited to US From UK
On June 14, a London court is expected to hold the first hearing on Assange's potential extradition to the United States.
Assange is currently serving 50 weeks in a London jail for violating his bail conditions in 2012, when he took refuge inside the Ecuadorian embassy in London to avoid extradition to Sweden on sexual assault charges.