WASHINGTON (Reuters) - U.S. prosecutors are preparing to pursue a criminal case against Wikileaks founder Julian Assange, escalating a long battle targeting his anti-secrecy group even as he remains holed up in the Ecuadorean embassy in London.

Prosecutors have obtained a sealed indictment against Assange, something that emerged in a Thursday filing in an unrelated criminal case in a Virginia federal court. Because the indictment was sealed, the nature of any charges against Assange has not been made public at this point.

Thursday’s filing had also been sealed, but it was made public for reasons that were unclear, according to a person familiar with the matter. Joshua Stueve, a spokesman for the U.S. attorney’s office in Alexandria, Virginia, said the filing was made in error.

Wikileaks said in a Twitter post it was an “apparent cut-and-paste error.”

Criminal charges in the United States could pressure Britain to extradite Assange, an Australian national. U.S. officials have acknowledged that federal prosecutors have been conducting a lengthy criminal probe into Assange and Wikileaks.

Assange could not be reached for comment.

Wikileaks became known over the last decade for publishing documents that were not previously public. In a recent high profile instance, during the 2016 U.S. presidential campaign the group published some of a trove of Democratic emails that U.S. intelligence agencies have concluded were hacked by Russia.

Intelligence agencies said Russia used hacking and other tactics to try to tilt the election in favor of Republican candidate Donald Trump. Moscow has denied this.

Lawyers for Assange and others have said his work with Wikileaks was critical to a free press and was protected speech.

“The notion that federal criminal charges could be brought based on the publication of truthful information is an incredibly dangerous precedent to set,” Barry Pollack, a U.S. lawyer for Assange, said in a statement.

Wikileaks said in a statement on Friday that Assange was willing to work with British officials as long he was not extradited to the United States.

FILE PHOTO - WikiLeaks founder Julian Assange is seen on the balcony of the Ecuadorian Embassy in London, Britain, May 19, 2017. REUTERS/Neil Hall/File Photo

Assange took refuge in the Ecuadorean embassy in 2012 to avoid extradition to Sweden for questioning over allegations of rape, which he has denied.

Sweden dropped its investigation of Assange, but Britain has said that if he left the embassy he would be arrested for violating his bail terms.

Greg Barns, an Australian lawyer advising Assange, said in a statement it was “no surprise” that the United States was seeking to charge Assange, and Australian officials should allow Assange to return there.

Ecuadorean officials had no immediate comment on Friday.

‘KEEP CONFIDENTIAL’

Thursday’s filing mentioning Assange related to a criminal case involving a 29-year-old man charged with enticing a 15-year-old girl.

The judge wrote in a detention memo that the defendant, Seitu Sulayman Kokayi, “has had a substantial interest in terrorist acts.” Reuters was unable to locate Kokayi.

A source familiar with the matter said the Koyaki case was totally unrelated to investigations of Assange or Wikileaks.

It was not clear why the filing referred to Assange, but it said that any procedure “short of sealing will not adequately protect the needs of law enforcement at this time because, due to the sophistication of the defendant, and the publicity surrounding the case, no other procedure is likely to keep confidential the fact that Assange has been charged.”

Wikileaks gained prominence in 2010 after publishing a classified video showing a 2007 U.S. helicopter attack in Iraq that killed a dozen people, including two Reuters news staff.

It has also released thousands of classified U.S. military documents, among other disclosures.

In April last year, when he was head of the CIA, U.S. Secretary of State Mike Pompeo called Wikileaks a “hostile intelligence service.”

Trump praised Wikileaks during his 2016 campaign.