Carl Bildt described suggestions by Ecuador that Assange was subject to political persecution and might be sent by Sweden to face prosecution in the United States over the Wikileaks website as "grave accusations".
He questioned why Assange, who has been accused of sexual assault by two women in Sweden, was not prepared to face justice there.
Australian-born Assange remains at the centre of a diplomatic spat involving six countries on five continents, having skipping bail to avoid extradition to Sweden.
He is currently holed up at the Ecuadoran embassy in London, which, under the terms of the Vienna Convention, British police can not enter.
William Hague, British Foreign Secretary, has refused him safe passage to leave the country, saying that Britain would abide by a European Arrest Warrant issued for his extradition by Sweden. He is liable for arrest if he steps out of the Ecuadoran Embassy and on to British soil.
Ricardo Patino, Ecuador's foreign minister, has said Assange fears "repression and intimidation" if he was sent to Sweden.
He added: "Ecuador is sure that there is a real threat of him being extradited to a third country, without any guarantees. He would be subject to cruel treatment."
In an interview on BBC Radio 4's World at One, Mr Bildt, a former prime minister of Sweden, described Mr Patino as living in a "fantasy world".
He went on: "The fact is that there is a case brought by two women of sexual assault. That has been taken out by the prosecutor who wants to ask Mr Assange what really happened.
"I fail to see why Mr Assange, if he is as innocent as he claims, doesn't answer to those requests.
"Some might consider it no crime and have a different view of these things but sexual assault is considered a serious crime."
"We were accused of some sort of political trial or political prosecution of Mr Assange.
"The UK and Sweden are probably among the two best countries in the world when it comes to the independence of the judiciary.
"I think honestly that we don't take too kindly to being lectured by the Ecuadorans."
Mr Pinto also said that Assange fears for his life if he is extradited to the United States, which still allows the use of the death penalty.
Mr Bildt pointed out that Swedish laws bars extradition in capital cases, describing Mr Pinto as "completely off the rails".
He added: "We can never extradite to any country where they face the question of the death penalty.
"Even the most [basic] reading of the Swedish legal system would have told the Ecuadoran foreign minister that."
Mr Bildt said Sweden had "complete confidence" in the United Kingdom's handling of the case.
The US currently has no case pending against Assange, who was given access to hundreds of thousands of top secret diplomatic cables by Bradley Manning, an Iraqi-based American soldier, but has refused to rule out doing so in future.
Manning is charged with 22 counts, including aiding the enemy, which can carry a sentence of death, but prosecutors have said that they will not seek his execution.