British Home Secretary Sajid Javid told the BBC that the final decision on Assange's extradition would be up to the court.
It was earlier reported that the US had formally submitted the extradition request to the UK for the WikiLeaks founder.
Assange faces an 18 count indictment that accuses him of soliciting and publishing classified information and of conspiring with former Army private Chelsea Manning to crack a Defense Department computer password.
That indictment, which includes Espionage Act charges, was issued by the Justice Department last month and is pending in a federal court in Alexandria, Virginia.
The extradition request had been expected ever since US authorities first announced a criminal case against Assange.
The 47-year-old was evicted on 11 April from the Ecuadorian Embassy in London, where he had been holed up since 2012 after Ecuador granted him political asylum. He was arrested by British police and is currently serving a 50 week sentence for jumping bail.
Sweden also seeks him for questioning about an alleged rape, which Assange has denied. Assange was initially charged in the US with a single computer crime violation on allegations that he worked with Manning to crack a government password.
Some legal experts have said the additional Espionage Act charges might slow or complicate the extradition process to the extent the UK views them as political offences and exempt from extradition.
Manning, who spent seven years in a military prison for delivering a trove of classified information to Assange before having her sentence commuted by then-President Barack Obama. She has been jailed for civil contempt in Virginia after refusing to testify before a grand jury investigating WikiLeaks.
The United States will detail all the charges against Assange when it seeks his extradition in a London court on Friday.
The "first real confrontation of arguments" in court will not be for several weeks or months, Mr Hrafnsson said.
The 18 charges against Assange reject his claim he was simply a publisher receiving leaked material - which would be protected under press freedom legislation.
