Former chief United Nations weapons inspector Hans Blix says he fears the United States has a secret plan to keep its troops in Iraq.

Dr Blix, who warned against the invasion of Iraq after his inspectors found no evidence that weapons of mass destruction were being held by Saddam Hussein, is in Sydney to receive the Sydney Peace Prize.

"One fear I would have is that the US has a hidden thought to remain in Iraq," Dr Blix told ABC radio on Wednesday.

"One reason why they wanted in was that they felt they must leave Saudi Arabia.

"After the Gulf War in 1991 they left their troops in Saudi Arabia to protect pipelines," he explained.