WikiLeaks founder Julian Assange has hit out at the US military, saying that it bears the ultimate responsibility for any deaths of Afghan informers in the wake of the publication by his organisation of 75,000 leaked files of American army secrets.

Assange and WikiLeaks, the whistleblowers' website that publishes leaked documents from around the world, have come under increasing fire amid accusations that publishing the files put people's lives at risk. But in an interview with the Observer, Assange said the blame for any deaths lay squarely with US military authorities.

"We are appalled that the US military was so lackadaisical with its Afghan sources. Just appalled. We are a source protection organisation that specialises in protecting sources and have a perfect record from our activities," he said.

WikiLeaks has been accused of disclosing the names of Afghan collaborators who may now be subject to reprisals. Critics also say that the information it published is unchecked and some of it may be of dubious provenance. But Assange responded to those claims by saying: "This material was available to every soldier and contractor in Afghanistan… It's the US military that deserves the blame for not giving due diligence to its informers."

Assange insisted there was no evidence that anyone had been put at risk and that WikiLeaks had held sensitive information back and taken great care not to put people at risk. "Well, anything might happen, but nothing has happened. And we are not about to leave the field of doing good simply because harm might happen… In our four-year publishing history no one has ever come to physical harm that we are aware of or that anyone has alleged."

However, he did concede that, if it was proven someone had been killed or injured because of the leak, then WikiLeaks would consider changing the way it operates. "We will review our procedures," he said. But that is unlikely to defuse the growing international row. Last week the Afghan president, Hamid Karzai, branded Assange "irresponsible". The US defence secretary, Robert Gates, said he might have "blood on his hands".

At the same time US authorities are broadening their investigation into how the leak happened. The suspected leaker, Private Bradley Manning, is in custody. He has already been charged with passing on a video shot in Iraq of a US helicopter attack and 150,000 classified diplomatic cables. He is also the main suspect in the Afghan "war logs" leak. Now, according to a report in the New York Times, investigators are probing whether Manning acted alone or with others. The focus of the inquiry was on a group of people in Cambridge, near Boston in Massachusetts, who might prove to be the link between Manning and WikiLeaks.

Assange said he was undeterred by the attacks, and that traditional journalism had vacated a space into which WikiLeaks was stepping. "We are creating a space behind us that permits a form of journalism which lives up to the name that journalism has always tried to establish for itself," he said.