Ecuadoran President Rafael Correa has slammed what he called Britain's "vulgar threats" to remove WikiLeaks founder Julian Assange from the Ecuador embassy in London where he has been granted asylum.

"Never, as long as I am president, will Ecuador accept threats like these, which are absolutely vulgar, inconsiderate and intolerable," Correa said in a weekly statement on Saturday.

"We will not give up our sovereignty. We respect everyone, and we are always seeking dialogue, but the final decision is ours."

On Wednesday, Ecuador said it had been informed in writing that Britain was willing to seize Assange from the embassy in order to extradite him to Sweden, where he is wanted for questioning about sexual assault allegations.

British Foreign Secretary William Hague said on Thursday that his government was obliged under its own laws to extradite the Australian national to Sweden.

"No one, least of all the government of Ecuador, should be in any doubt that we are determined to carry out our legal obligation," Hague told reporters.

'No safe passage'

Assange says he fears Sweden will extradite him to the United States, a claim the US has denied, which views WikiLeaks as a threat to its national security following the leak of a trove of secret front-line military reports and confidential embassy cables.

Assange took refuge in the Ecuadorian embassy in London on June 19 after exhausting all legal appeals against the extradition.

Ecuador has granted Assange's request for asylum, but with British police posted outside the embassy, it is unclear whether he will be able to leave the building.

Hague said on Thursday that Britain would not grant Assange safe passage because "there is no legal basis for us to do so".

Assange plans to make a public statement on the case on Sunday at 1300 GMT, according to a spokesman.

The Organisation of American States has, meanwhile, called a meeting of foreign ministers for August 24 to discuss the Assange case and the widening diplomatic row between Quito and London.