The internet broadcast of video footage showing the decapitation of a US hostage by Iraqi militants signals a new era in wartime propaganda, according to war historian Phillip Knightley.

"I can't think of any war in living memory where images have been so important," said Knightley, author of "The First Casualty", an acclaimed history of war correspondents and propaganda.

"Usually the underdog has not been able to manipulate the media in the same way that the major Western news corporations have," Knightley said on Australian Broadcasting Corporation radio.

"But now they've learned how to do it and anyone with a small digital camera that is prepared to take images that are so horrific that they can't be ignored is certain of worldwide coverage," he said.

The images released this week on an Islamic website of the beheading of American Nick Berg by Iraqi militants and leaked pictures of US soldiers abusing Iraqi prisoners are a "landmark in the whole history of the way wars are covered", he said.

"These images are increasingly horrific and this may not be a bad thing because for the first time the ordinary viewing and reading public are learning what the true face of war is really like," he said.

"Up until now the western media ... have been quite good at portraying the shock and awe of the American attack on Iraq, but they have been less good at portraying the shocked and the awed," he said.

Knightley said with the advent of the internet there was no way to control the proliferation of war images, while the development of Arab news channels had helped to "lift the veil" of the horror of war.

"The risk is that in the meantime, we will face atrocity versus counter atrocity, then another atrocity and a sort of descent into barbarity," he said.

The Sydney-born Knightley worked for The Sunday Times in London for 20 years where he won acclaim for uncovering the dangers of thalidomide.

He has also written books on KGB double agent Kim Philby, Lawrence of Arabia and the Profumo scandal. - Sapa-AFP