The United Kingdom government has refused to let WikiLeaks founder Julian Assange receive medical attention outside Ecuador's London embassy where he has been holed up for more than three years.

Ecuador's Foreign Minister Ricardo Patino made the claim when he said he asked the UK for a special "safe-passage" so that Assange, 44, could have a magnetic resonance imaging scan after suffering "deep pain" in his right shoulder for the past three months.

"The (UK) government is not offering the terms to make this happen," Patino said. "It's an additional fault in his protection, in the defense of a person's human rights. We don't know what he may have, and they don't want to give an authorization that they can perfectly well give."

Assange, who is Australian, entered the embassy in June 2012 to avoid being extradited to Sweden where he is wanted for questioning over a rape allegation, which he denies.