Julian Assange will not leave Ecuador's embassy even if Sweden drops its extradition bid over accusations of sexual assault, because he fears moves are already underway by the US to prosecute him on espionage charges, he has said.

On the eve of the anniversary of his seeking asylum in the embassy in Knightsbridge, Assange said he believed a sealed indictment had already been lodged by a grand jury in Virginia, which could see him being arrested and extradited by Britain to the US to face prosecution over the WikiLeaks cable releases.

"The strong view of my US lawyer is that there is already a sealed indictment, which means I would be arrested, unless the British government gave information or guarantees that would grant me safe passage," the WikiLeaks founder told a small group of news agencies.

"We know there is an ongoing investigation in the US and we know I am a target of the federal grand jury. There is a 99.97% chance that I will be indicted. So if the Swedish government drops their request [to go to Sweden] tomorrow, I still cannot leave the embassy. My lawyers have advised me I should not leave the embassy because of the risk of arrest and extradition to the US."

The foreign secretary, William Hague, and his Ecuadorean opposite number, Ricardo Patiño, met on Monday to discuss the ongoing diplomatic stalemate, but were unable to reach agreement. Ecuador argues that having been granted political asylum, Assange should be allowed to board a plane to Quito unimpeded but the British authorities have insisted they will not let him leave without acting on the Swedish warrant. The two countries agreed to set up a legal working group in an attempt to arrive at a solution to the impasse.

Asked if he regretted seeking asylum because of the resulting stalemate, Assange said: "Strategically, it has been exactly what I hoped for."

Living and working from a small room in the embassy was less stressful than the 600 days he spent on bail wearing an electronic tag, Assange said, although he admitted that it took "diligence" to stay healthy with limited access to natural light. "You can get rickets by not having any sunlight – it is not healthy to be in this position."

But he shrugged off reports that he had been ill during his time in the embassy, describing health stories about a chest condition as "lung-gate". Assange said that when he sought asylum he had believed he would be in the embassy for between six months and two years, a timescale he still considered realistic.

"My case could be swiftly resolved if Sweden were to guarantee that I would not be extradited to the US or if the British government would guarantee to veto any such extradition to the US," he said.

"While I remain hopeful that a diplomatic solution can be reached, or that the Swedish and US authorities will cease their pursuit of me, it remains the case that it is highly unlikely that Sweden or the UK will ever publicly say no to the US in this matter."