The claim came on a website posting, but no pictures were provided to support the statement.
A group statement posted on the internet said the two were "executed" after US forces failed to release Sunni women prisoners as demanded by the group.
But the US military denied an al-Qaida claim that rebels had abducted two marines whom they threatened to kill.
"Claims that two US marines were kidnapped in western Iraq 2 October are patently false," a statement said.
"All marines and service members ... are accounted for," it added.
Elsewhere in Iraq, an American soldier died of his injuries after a combat patrol struck a mine near Baji on Saturday, the US military said on Monday.
Another US soldier was killed on Saturday when a patrol struck an improvised explosive device in central Baghdad.