Ecuador is "very concerned" about the health of Julian Assange after the WikiLeaks founder lost a lot of weight while staying at the country's embassy in London, a foreign ministry official said while in Moscow.

The deputy foreign minister of Ecuador, Marco Albuja, expressed his concern for Assange as he gave a briefing to Russian press after wrapping up his delegation's visit to Russia.

"Assange has visibly lost weight, and we are very concerned for his health," he said, quoted by the Voice of Russia radio. "In case of his illness we will have to pick among two options: to treat Mr Assange at the embassy or to hospitalise him."

Ecuador has asked the British government for written assurances that Assange, who has been granted asylum by Quito and remains holed-up in the embassy building in London, will not be arrested in the event of hospitalisation.

"So far Britain has not agreed to this request but is thinking it over," Albuja said.

Britain's Foreign Office said it had not yet received a request.

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

The Ecuadoran embassy in Moscow said Wednesday that the delegation was in Moscow to meet with Russian foreign ministry officials and discuss bilateral affairs such as the flower trade.

Former computer hacker Assange, 41, walked into the London embassy on June 19 seeking asylum in a bid to avoid extradition to Sweden, where he faces questioning over alleged rape and sexual assault.

He was granted asylum on August 16 but Britain refuses to grant him safe passage out of the country, and he remains in the embassy, putting Ecuador in a diplomatic stalemate with Britain.

Assange said in an interview published September 30 that his health was "slowly deteriorating" in the embassy, adding that he had "a racking cough".

He said he was keeping fit by using a running machine, boxing and seeing a personal trainer -- reportedly an ex-soldier in Britain's elite SAS who is now a whistleblower -- every other day.

Assange denies the sex crimes allegations and claims he could eventually be passed from Sweden to the United States for prosecution over the WikiLeaks website's publication of hundreds of thousands of classified US documents.

WikiLeaks enraged Washington in 2010 by publishing a flood of secret military files on the wars in Iraq and Afghanistan, as well as a huge cache of diplomatic cables from US embassies across the world.