The government of Ecuador has voiced concerns about the "serious lack of progress" by the Swedish authorities in dealing with WikiLeaks founder Julian Assange.

A letter has been sent to the Swedish government saying there has been a "serious failure" by the country's prosecutor, including a "lack of initiative" to complete inquiries.

The Australian has been living inside the Ecuadorian Embassy in London for almost five years and has been granted political asylum.

He was questioned six months ago in the presence of Swedish officials over a sex allegation, which he has always denied.

Assange faces extradition to the United States for questioning over the activities of WikiLeaks if he leaves the embassy.

The letter raises developments in the United States since the election of Donald Trump as president, including a speech by CIA director Mike Pompeo describing WikiLeaks as a "hostile intelligence service".

Recent public declarations such as this constituted an "obvious risk" for Assange, said the letter.

The Ecuadorian government reiterated that Assange was under international protection inside the embassy.

The letter expresses "great concern" that six months after being questioned, the Swedish prosecutor has not issued any declaration on the procedural situation.

It repeated Ecuador's request for guarantees of non-extradition to any third country.

The letter, from Ecuador's Foreign Affairs Ministry, urges the Swedish authorities to resolve Assange's situation as quickly as possible.