"My lawyers have advised me I should not leave the embassy because of the risk of arrest and extradition to the US," Assange told reporters in Londan at the embassy.
"The strong view of my US lawyer is...I would be arrested, unless the British government gave information or guarantees that would grant me safe passage. We know there is an ongoing investigation in the US and we know I am a target of the federal grand jury," he said.
"There is a 99.97 per cent chance that I will be indicted. So if the Swedish government drops their request tomorrow, I still cannot leave the embassy," he added.
Assange, 41, had sought asylum in the embassy a year ago after he lost an appeal against extradition to Sweden for questioning over alleged sex offences he has always denied.
The Australian national now says he still fears being sent to the US for releasing thousands of secret diplomatic documents including 250,000 State Department cables. US Army soldier Bradley Manning has admitted passing those documents to WikiLeaks.
UK foreign secretary William Hague has told British MPs that there was no progress on the case during talks with his Ecuadorian counterpart, Ricardo Patino, earlier this week.
Assange said he always believed it could take between six months and two years to resolve his case.
He claimed there was a sealed indictment already lodged against him in the US which would lead to him being taken from the UK if he stepped outside the embassy.
Assange was granted asylum by the Ecuadorian government after a final appeal against his extradition was turned down by the UK Supreme Court.
According to the BBC, Assange pointed out that he had effectively been under house arrest for almost 600 days as he fought extradition to Sweden through the courts and had to wear an electronic tag on his ankle and report regularly to the police.
"It is less stressful in the embassy," he said, although he said there could be long-term physical problems to staying indoors for so long.
He has a sun lamp and treadmill as well as a computer where he says he spends 17 hours a day working.
The UK government has said it has a legal obligation under European law to see that Assange is sent to Sweden where prosecutors have wanted to question him since 2010.
"It is legally bound, under higher laws which it has acceded to, as part of its United Nations obligations, to accept the transfer of political refugees to the country which granted asylum.
"That the UK government chooses to ignore its obligations under international law and instead favour an interpretation of EU law is, of course, political," Assange countered.
London's Metropolitan Police has estimated the cost of policing the Ecuadorian embassy between June 2012 and May 2013 at around 3.8 million pounds.
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