The WikiLeaks founder, Julian Assange, is tonight facing growing legal problems around the world, with the US announcing that it was investigating whether he had violated its espionage laws.

Assange's details were also added to Interpol's worldwide wanted list. Dated 30 November, the entry reads: "sex crimes" and says the warrant has been issued by the international public prosecution office in Gothenburg, Sweden. "If you have any information contact your national or local police." It reads: "Wanted: Assange, Julian Paul," and gives his birthplace as Townsville, Australia.

Friends said earlier that Assange was in a buoyant mood, however, despite the palpable fury emanating from Washington over the decision by WikiLeaks to start publishing more than a quarter of a million mainly classified US cables. He was said to be at a secret location somewhere outside London, along with fellow hackers and WikiLeaks enthusiasts.

In contrast to previous WikiLeaks releases, Assange has, on this occasion, kept a relatively low profile. His attempt to give an interview to Sky News via Skype was thwarted today by a faulty internet connection.

But he was able to give an interview to Time magazine in which he called for Hillary Clinton, the US secretary of state, to resign. "She should resign, if it can be shown that she was responsible for ordering US diplomatic figures to engage in espionage in the United Nations, in violation of the international covenants to which the US has signed up. Yes, she should resign over that," he said.

Assange's reluctance to emerge in public is understandable. It comes amid a rapid narrowing of his options. Several countries are currently either taking – or actively considering – aggressive legal moves against him. This lengthening list includes Sweden, Australia and now the US – but so far as can be made out, not Britain.

The US attorney general, Eric Holder, announced yesterday that the justice department and Pentagon are conducting "an active, ongoing criminal investigation" into the latest Assange-facilitated leak under Washington's Espionage Act.

It was not immediately clear whether Holder was referring to Bradley Manning, the dissident US private suspected of being the original source of the leak, or Assange. The inquiry by US federal authorities is made tricky by Assange's citizenship – he is Australian – and the antediluvian nature of the law's pre-internet-era 1917 statutes.

According to the Washington Post, no charges against anyone from WikiLeaks are imminent. But asked how the US could prosecute Assange, a non-US citizen, Holder struck an ominous note. "Let me be clear. This is not sabre-rattling," he said, vowing to swiftly "close the gaps" in current US legislation.

But Assange's most pressing headache is Sweden. Swedish prosecutors have issued an international and European arrest warrant (EAW) for him in connection with rape allegations, and the warrant has been upheld by a Swedish appeal court.

Assange strongly denies any wrongdoing but admits having unprotected but consensual encounters with two women during a visit to Sweden in August.

Mark Stephens, his London-based lawyer, has described the allegations as "false and without basis", adding that they amount to persecution as part of a cynical smear campaign.

Nonetheless, the Swedes appear determined to force Assange back to Sweden for questioning. Stockholm's director of public prosecutions, Marianne Ny, said last month: "So far, we have not been able to meet with him to accomplish the interrogation."

Assange contests this too. But if he declines to return to Sweden voluntarily, and the UK decides to enforce Sweden's arrest warrant, things may get tricky. Some friends believe Assange's best strategy is not to go to ground but to get on a plane to Sweden and face down his accusers.

Stephens, moreover, says that the Swedish attempts to extradite Assange have no legal force. So far he has not been charged, Stephens says – an essential precondition for a valid European arrest warrant.

Under the EAW scheme, which allows for fast-tracked extradition between EU member states, a warrant must indicate a formal charge in order to be validated, and must be served on the person accused.

"Julian Assange has never been charged by Swedish prosecutors. He is formally wanted as a witness," Stephens told the Guardian today.

"All we have is an English translation of what's being reported in the media. The Swedish authorities have not met their obligations under domestic and European law to communicate the nature of the allegations against him in a language that he understands, and the evidence against him."

Assange's legal team are challenging the warrant in Sweden's supreme court. They are optimistic: a previous appeal was partially successful in limiting the grounds on which the warrant was issued.

Today a spokesman for Britain's Serious Organised Crime Agency, which is responsible for validating extradition requests, would not confirm or deny receipt of a European arrest warrant for Assange's extradition.

Assange has previously suggested he might find sanctuary in Switzerland. More promising perhaps is Ecuador, whose leftist government unexpectedly offered him asylum on Monday.

"We are ready to give him residence in Ecuador, with no problems and no conditions," Ecuador's foreign minister, Kintto Lucas, said.

At the very least, Ecuador could offer Assange a new passport. He might need one. Yesterday Australia's attorney general, Robert McClelland, said Australian police were also investigating whether any Australian laws had been broken by the latest WikiLeaks release.

In reality, Assange's predicament may not be as hopeless as it seems. The US would be hard pressed to make charges against him stick, experts suggest.

"There have been so few cases under the Espionage Act, you can put them on one hand," said David Banisar, senior legal counsel for the campaigning group Article 19 and an expert on free speech in the US. "There is the practical problem that most of the information published by WikiLeaks wasn't secret. Then there is the debate about whether the documents were properly classified – there are detailed rules in the US about what can and cannot be classified."