The WikiLeaks founder has been holed up in the Ecuadoran embassy in London since 2012, seeking to avoid extradition to Sweden.
Swedish prosecutors want to question Assange about a rape claim, which carries a 10-year statute of limitations that expires in 2020.
Assange, who faces arrest if he tries to leave the embassy, denies the allegation and insists the sexual encounter was consensual.
“We did ask the British government for a safe passange for humanitarian reasons in coordination with Ecuador, so that Julian Assange can get an MRI,” Foreign Minister Ricardo Patino told a briefing in Quito on Wednesday.
“The reply we have had from Britain is that he can leave whenever he likes for any medical care he might need, but the European arrest warrant for Assange is still valid.In other words, he can leave – and we will put him in jail,” Patino added.
Patino stressed that Quito was not renewing a prior request for Assange to be able to travel to Ecuador.
“We are requesting a special safe passage permission that would last just a few hours, just so that he can go and get an MRI (magnetic resonance imaging) and come back,” Patino added.
The foreign minister spoke two days after Britain said it would stop continuously standing guard outside the Ecuadoran embassy in London.
The 44-year-old Australian also fears that if he leaves he could eventually face extradition to the United States and a trial over the leak of hundreds of thousands of classified military and diplomatic documents in 2010.
Patino earlier said that “the British police have no reason to spend so much money to deploy so many police and vehicles outside the embassy”.
He recalled that Sweden and Ecuador are negotiating what he called an international criminal assistance accord and said no deadline for the agreement has been set.