Wikileaks founder Julian Assange, who published leaked US diplomatic cables, fears he could face the death penalty in the US, defence documents say.
He is fighting extradition from the UK to Sweden over allegations, which he denies, of sexual offences against two women.
His lawyer said there was a "real risk" the US would then seek extradition over the leaking of the diplomatic cables.
The claim was made ahead of a full extradition hearing next month.
Mr Assange founded the whistle-blowing website which published thousands of US embassy cables and other confidential documents online, prompting the US to examine possible charges against him.
He appeared before District Judge Nicholas Evans for a 10-minute hearing on Tuesday at Belmarsh Magistrates' Court, sitting at Woolwich Crown Court.
The 39-year-old Australian spoke only to confirm his name, age and address, and his case was adjourned until 7 and 8 February for a full hearing.
The defence document, posted on the Finers Stephens Innocent website at the request of Mr Assange, suggested that extraditing him to Sweden could breach Article 3 of the European Convention on Human Rights (ECHR), which bans torture.
"It is submitted that there is a real risk that, if extradited to Sweden, the US will seek his extradition and/or illegal rendition to the USA, where there will be a risk of him being detained at Guantanamo Bay or elsewhere, in conditions which would breach Article 3 of the ECHR," the document said. BBC News
