A hidden microphone was found in Ecuador's embassy in London, where WikiLeaks founder Julian Assange is holed up, Ecuador's foreign minister said.

Ricardo Patino said the origin of the bug would be revealed Wednesday and explanations demanded of the country involved. He did not elaborate.

The bug was found in a routine search ahead of his visit to London on June 16, he told reporters.

The microphone was found in the office of the ambassador Ana Alban.

Patino said he was not insinuating that the bug was linked to the saga surrounding fugitive US intelligence leaker Edward Snowden, who has applied for asylum in Ecuador.

Snowden is wanted by the United States on charges of espionage for revealing a massive phone and Internet surveillance programme.

Assange, whose organization leaked a vast trove of diplomatic cables and Iraq and Afghanistan war logs a few years ago, has been staying in the Ecuadoran embassy in London for a year.

He faces questioning over sexual assault allegations in Sweden, and fears the Swedes will hand him over to the Americans.

During his visit to London, Patino met with his British counterpart William Hague but no agreement on the Assange case was reached.

Britain is refusing to grant Assange safe passage out of the embassy so he can travel to Ecuador.