A second veto would send amended law back to parliament for a new round of voting.
The officer, who was also a policeman during Saddam’s rule, had previously escaped several assassination attempts.
In another incident in northern Iraq, two soldiers were killed at a checkpoint in the town of Mohallabiyah, 40km northwest of the main northern city of Mosul, when assailants opened fire on them before fleeing.
In Baghdad, one person was killed and six wounded by a bomb in Adhamiyah, a predominantly Sunni Arab northern neighbourhood, an interior ministry official said.
Violence across Iraq dropped dramatically last month, with the fewest deaths in attacks since the US-led invasion of 2003.
Official figures showed that a total of 122 people were killed last month – 88 civilians, 22 policemen and 12 soldiers.