Operation Phantom Strike was being staged throughout Iraq, specifically targeting al-Qaeda-linked militants and also Iranian-backed groups, it said.
The move comes as Iraqi leaders are preparing for emergency talks called on Sunday by Prime Minister Nouri Maliki.
Mr Maliki urged Iraq's main political factions to hold talks in an attempt to break the country's political crisis.
In recent weeks almost all Sunni members of the cabinet have quit. Others are boycotting meetings, leaving at least 17 cabinet seats empty.
'Simultaneous operations'
The new assault would build on the successes during recent offensives in the capital Baghdad and suburbs, the US military said.
"It consists of simultaneous operations throughout Iraq focused on pursuing remaining AQI (al-Qaeda in Iraq) terrorists and Iranian-supported extremists elements," it said.
Lt Gen Ray Odierno, the US second-in-command in Iraq, said that his aim was "to continue to pressure AQI and other extremist elements throughout Iraq".
US officials have repeatedly accused Iran of arming, funding and training Iraqi Shia insurgents. Tehran denies the claims.
The US military declined to elaborate on the offensive, citing "security concerns".
The operation was launched a day after up to five militants were killed and 13 others were injured during a raid on Baghdad's Shia district of Sadr City.
Maliki's warning
On Sunday, Mr Maliki urged Iraq's main political leaders to hold an urgent meeting to discuss how to end a political paralysis gripping the country.
A senior Kurdish leader, Massoud Barzani, has already arrived in Baghdad for the talks.
He will try to get the Sunni parties to decide whether to rejoin the government or go into opposition.
Mr Maliki has been unable to push forward with his plans for national reconciliation without the support of the country's various factions.
The so-called national unity government has effectively disintegrated, says the BBC's Richard Galpin in Baghdad.