A major international trade fair, which was due to open in the Iraqi capital, Baghdad, on Monday, has been postponed.
The move follows the killing of four Americans in the town of Fallujah earlier this week.
More than 200 companies from 26 countries were due to exhibit at the trade fair, but American officials said the security of those attending could not be guaranteed.
Television pictures of the Fallujah massacre have continued to cause shock and revulsion in the US.
The senior US official in Iraq, Paul Bremer, said there would be a robust response to the attack.
