Lieutenant Colonel Robert Strzelecki said Sunday's powerful blast destroyed the depot in a desert area 180km southwest of Karbala, and it was impossible to immediately confirm how many died.
"Some troops were sent to this area to protect it and secure it and retrieve the bodies," said Strzelecki from the multinational force headquarters in Hilla, south of Baghdad.
He said the raiders were spotted on a radar screen, but it was difficult to gauge how many were in the group.
"There was a lot of dust and damage. We don't have the exact number. It's difficult to say because on the screen we saw a group of people enter the bunker. It could be five, 10 or 15 people," he said.
"There was a minimum of four or five. For sure, all of them were killed," he said, adding there were no occupation forces harmed in the blast.
The Polish news agency PAP earlier quoted Strzelecki saying there could be 20 killed in the blast.
The depot, previously used by the army of deposed President Saddam Hussein, comprised about 100 buildings under the guard of 80 Polish soldiers.
Munitions dumps across Iraq are regularly targeted by looters who dismantle the weapons to recycle brass casings and explosive contents.
Poland heads up a 9000-strong multinational force in Iraq made up of largely Polish, Spanish and Ukrainian troops, alongside soldiers from 20 other countries.