In a letter to the Swedish government, Ecuadorian authorities accused the Swedish prosecutor in the case of a "lack of initiative," the BBC reports.
Ecuador believes the prosecutor's inability to complete an investigation into a alleged rape in Stockholm dating back to 2010 to be a "serious failure", according to the letter.
The WikiLeaks founder is currently remanded in custody by Sweden 'in absentia' over the 2010 allegation. Swedish prosecutor Ingrid Isgren travelled to London at the end of 2016 in order to attend while he was interrogated by an Ecuadorian prosecutor regarding the allegation, which he has always denied.
The Australian has been taking refuge in the Ecuadorian embassy in London for five years now, refusing to return to Sweden to be questioned. He argues that there is a risk the Nordic nation will extradite him to the US over WikiLeaks' release of 500,000 secret military files.
Earlier in May his lawyer filed a request at Stockholm District Court asking for an end to the arrest warrant against his client in Sweden, arguing it should be dropped after the US openly declared its intent to imprison him. US attorney general Jeff Sessions has said that arresting the WikiLeaks founder is "a priority".
British police meanwhile have said that Assange will be arrested if he leaves the Ecuadorian embassy in London.