Assange, who is Australian, has been living at the embassy since June 2012 when he was granted diplomatic asylum.
Last week, Swedish prosecutors offered to travel to the embassy in London to question Assange over sex crime allegations, dropping their previous insistence that Assange travel to Sweden to be interrogated about a 2010 incident.
He refused to return to Sweden, where he says he would be arrested and extradited to the US. Assange denies the allegations, which are not related to WikiLeaks' publication of US military and diplomatic documents five years ago.
Michael Ratner, a US lawyer who represents Assange and WikiLeaks, said if Assange left the embassy, he was likely to be arrested by British authorities and risked being extradited to America.
"Even were the Swedish case to be disposed of, the UK would arrest Assange upon leaving the embassy for claimed violations of bail conditions or something similar," Ratner told Reuters, adding that Washington would almost certainly seek his extradition.
Earlier this month, a ruling from a Washington federal judge revealed that the US Department of Justice and the FBI remain engaged in a criminal investigation into WikiLeaks.
Ratner said that investigation has been going on for "at least five years."
Another legal source close to Assange told Reuters that he would remain in the Ecuadorian Embassy until the US ended a grand jury investigation of WikiLeaks.
A spokesman for the US Attorney's office in Alexandria, Virginia, said the investigation into WikiLeaks remained open. Other US law enforcement sources said criminal charges had not yet been filed against Assange.
The sex allegations against Assange were lodged against him by two female WikiLeaks supporters who hosted him during a visit to Sweden.
The statute of limitations on several of the crimes of which Assange is suspected runs out in August 2015 – a factor that influenced Swedish prosecutors’ decision to interview Assange at the embassy.
British authorities have declined to say what they might do if Sweden were to close its investigation of Assange and he were to try to leave the Ecuadorian Embassy in London.