President Barack Obama should resign if it can be shown that he approved spying by US diplomatic figures on UN officials, the founder of WikiLeaks has said.

"The whole chain of command who was aware of this order, and approved it, must resign if the US is to be seen to be a credible nation that obeys the rule of law. The order is so serious it may well have been put to the president for approval," Julian Assange told Spanish daily El Pais.

"Obama must answer what he knew about this illegal order and when. If he refuses to answer or there is evidence he approved of these actions, he must resign," he added during an Internet chat interview published online yesterday.

WikiLeaks threw US diplomacy into chaos when it started releasing more than 250,000 classified State Department cables on November 28, creating an international firestorm as American diplomats' private assessments of foreign leaders and politics have been publicly aired.

According to one of the documents, US Secretary of State Hillary Clinton asked for UN personnel's telephones, emails, credit card details and frequent flier numbers.

The United States and other governments said the release of the documents broke their laws.

Assange gave the interview to El Pais on Saturday from an undisclosed location. The 39-year-old Australian is believed to be in Britain, and a report said he could be arrested this week.

WikiLeaks has come under intense pressure to close since it began releasing the trove of US State Department cables.

The site has already been forced to change its domain name and hop-scotch to servers around the globe as successive companies and countries bent to US arguments branding its divulgations over the past week "illegal".

It has also come under repeated cyber-attack, through a tactic known as distributed denial of service (DDoS) in which thousands of computers connect to its servers in a concerted attempt to knock them off-line.

Mirror websites, which replicate WikiLeaks's data, have sprung up on servers in various countries.

Interpol, meanwhile, has issued a "red notice" against Assange alerting all police forces that he is a wanted person in Sweden, which wants to question him "in connection with a number of sexual offenses", charges he denies.

"The organisation is strong. We have a lot of support, however we also have many attacks of different forms. From ongoing mass DDoS attacks to smears and the legal issues," said Assange.