He was speaking in Baghdad on Wednesday hours after guerrillas fired rockets at the fortified compound where he was staying.
Straw, on a previously unannounced visit, told reporters he had been unaware of the blasts that echoed across Baghdad on Tuesday night and prompted an alarm in the sprawling compound of the US-British administration.
"Attack. Take cover. This is not a test," blared loudspeakers at the compound in one of Saddam Hussein's former palace complexes. Sirens wailed, flares lit up the night sky and U.S. helicopters clattered overhead.
In October, guerrillas rocketed a strongly fortified hotel inside the compound where U.S. Deputy Defense Secretary Paul Wolfowitz was staying. One U.S. soldier was killed.
A spokesman for the U.S. 1st Armored Division said at least two rockets had been fired in Tuesday's attack in Baghdad, but caused no casualties.
Two Iraqi police were wounded in a separate rocket-propelled grenade attack near a petrol station.