WikiLeaks' Australian founder Julian Assange is free to leave Sweden, after prosecutors said there's no arrest warrant against him for an alleged case of rape, one of his lawyers says.

Bjorn Hurtig said on Saturday an investigation is still under way, but the head of the whistleblowing website has been given no summons for questioning.

"I have been told that there is no arrest warrant against him", meaning Assange can do what he likes, including going abroad, Hurtig said.

Sweden's director of prosecutions Marianne Ny said on September 1 she was reopening a rape probe against the Australian, who had an arrest warrant against him issued on August 20 but saw it withdrawn by another prosecutor the following day.

Assange, 39, has said the allegations against him are part of a "smear campaign" aimed at discrediting his website, which is locked in a row with the Pentagon over the release of secret US documents about the war in Afghanistan.