Ecuador says it is concerned about the health of WikiLeaks founder Julian Assange and has asked Britain to guarantee him safe passage to a hospital if he requires treatment.

Mr Assange has been holed up inside Ecuador's embassy in London since June to avoid extradition to Sweden, where he is wanted for questioning over sexual assault allegations.

British authorities say Mr Assange will be arrested if he sets foot outside the embassy, which is under constant police surveillance.

"Assange has grown noticeably thinner, and we are very concerned about his health," Voice of Russia radio quoted Ecuador's deputy foreign minister Marco Albuja Martinez as saying.

"If he falls ill, we will have to choose between two alternatives: to treat Assange in the embassy or hospitalise him," Mr Albuja Martinez added.

"This is a very serious situation and it can affect Assange's human rights."

The comments were confirmed by the Ecuadorean embassy in Moscow.

Ecuador has asked the British Foreign Office for a document that would enable Mr Assange to enter hospital safely if necessary and return to the embassy with refugee status, the Voice of Russia quoted Mr Albuja Martinez as saying.

Britain's Foreign Office said it was unaware of Mr Assange's health problems.

"Ecuador have not told us that Mr Assange is ill. However, were they to do so, we would consider the matter," a Foreign Office spokesman said.

Ecuador granted Mr Assange asylum in August, saying it shared his fears that he could face charges in the United States over the publication by WikiLeaks in 2010 of thousands of secret US diplomatic cables.

When he appeared on a balcony of the building to address supporters in August, Mr Assange appeared tanned and in good health, but a BBC reporter who saw him recently described him as "a very pale man" in a story broadcast on Sunday.

Mr Assange broke the conditions of his bail when he entered the embassy after running out of legal options to avoid being sent to Sweden.

Speaking about the safe passage request he said Ecuador had lodged with the Foreign Office, Mr Albuja Martinez said his country was pleased that Britain "did not reject it outright".

"We will not put pressure on them and will patiently await an answer, so that Assange can receive medical treatment if necessary," he was quoted as saying in Moscow.

Reuters/ABC