The threat to appeal to the U.N. court, known officially as the International Court of Justice, followed Britain's promise to arrest Assange if he tries to leave the Ecuadorean Embassy in London.
He has been holed up in the redbrick embassy near Harrods department store since June 19 in a bid to avoid being extradited to Sweden, where he faces allegations of sexual assault.
"We will not allow Mr. Assange safe passage out of the United Kingdom, nor is there any legal basis for us to do so," British Foreign Secretary William Hague said Thursday.
Britain Wednesday threatened it would use a rarely cited law to revoke the embassy's diplomatic protection, and would barge into the embassy if Ecuador did not hand over Assange, the BBC and the British newspaper The Daily Telegraph reported.
Ecuador condemned the threat as a "complete intimidation."
The 1961 Vienna Convention on Diplomatic Relations says a host country may not enter the premises of an embassy without the represented country's permission. But embassies are not fully exempt from the jurisdiction of the countries they're in and are not sovereign territory of the represented state.
WikiLeaks said Thursday Assange, 41, would give a "live" media statement "in front of" the Ecuadorean Embassy Sunday, when he will have been at the facility for two months. He could be seized if Britain deems he has stepped outside the building's normally diplomatically protected zone.
British authorities intensified a police presence around the embassy Wednesday. About three dozen elite and other police units surrounded the embassy early Friday.
The Union of South American Nations, modeled on the European Union, said it would convene an "extraordinary meeting" in Ecuador Sunday to discuss the embassy crisis.
Assange's lawyer -- former top Spanish judge Baltasar Garzon, who came to international attention in 1998 when he indicted Chilean ex-dictator Augusto Pinochet -- said Britain was acting far beyond its authority because Assange was a political refugee accepted for asylum by a sovereign nation and Britain was obligated to honor that.
"They have to comply with diplomatic and legal obligations under the 1951 U.N. Convention Relating to the Status of Refugees, and respect the sovereignty of a country that has granted asylum," he told the Spanish newspaper El Pais.
The refugee convention defines who is a refugee, and sets out the rights of individuals granted asylum and the responsibilities of nations that grant asylum. It provides for special travel arrangements for refugees granted asylum under the convention.
"If Britain doesn't comply with its obligations, we will go before International Court of Justice to demand that Britain complies with its obligations because there is a person who runs the risk of being persecuted politically," Garzon told El Pais.
Assange has argued, and Ecuador Thursday agreed the evidence was strong, that the extradition to Sweden was a pretext for Assange to be sent to the United States, where authorities were incensed by WikiLeaks' 2010 release of 391,832 secret U.S. documents on the Iraqi war and 77,000 classified Pentagon documents on the Afghan conflict.
The whistle-blowing Web site also made available about 250,000 confidential diplomatic cables between the U.S. State Department and more than 270 U.S. diplomatic outposts around the world.
Unconfirmed reports cited by The New York Times indicate a secret grand jury hearing in Alexandria, Va., was considering a U.S. Justice Department bid to charge Assange with espionage.
Leaked e-mails from Strategic Forecasting Inc., a global intelligence company commonly known as Stratfor, suggest a sealed indictment is ready to be made public when U.S. officials determine the legal proceedings against Assange in Britain and Sweden have come to a close.
Assange claims Washington may want to execute him for "political crimes" associated with exposing government secrets.
He denies 2010 allegations of sexual molestation, coercion and rape made by two women in Stockholm, where he was promoting his Web site. He is wanted for questioning on one count of unlawful coercion, two of sexual molestation and one of rape.
Swedish prosecutors say they have solid case against Assange, but have not charged him with a crime.