"Three US marines were killed in action and two others wounded on 31 January while conducting security and stability operations in northern Babil province," the statement said on Monday.
The latest deaths marked the first fatalities reported by the US military since poll centres closed in Iraq's elections on Sunday.
The deaths raised the overall toll for US soldiers killed in Iraq to 1435 since the US-led invasion of March 2003, according to Pentagon figures.
Separately, a US military base in the former presidential complex in Mosul came under heavy mortar attack, Aljazeera learned. Columns of smoke were seen rising over the base.
Civil war fears
Meanwhile, Iraqi interim Vice-President Ibrahim al-Jaafari warned of the possibility of civil war breaking out if US troops pulled out prematurely.
On Monday al-Jaafari, who heads the Islamic Dawa (Propagation) Party, said: "We are doing our best to save the country from civil war.
"The withdrawal of the multinational troops from Iraq at this stage would trigger further killings, explosions and victims."