The US military said yesterday that about 75 militants were killed in the first 24 hours. Marines, sailors and soldiers from the 2nd Marine Division, are conducting the offensive in an area north of the Euphrates River, in the al-Jazirah Desert, a known smuggling route and sanctuary for foreign insurgents.
The brief statement did not specify when the operation began, how many troops were involved or whether there had been any American casualties.
A journalist embedded with the US forces reported that more than 1,000 US troops supported by fighter jets and helicopter gunships on Sunday raided villages in and around Obeidi, about 185 miles west of Baghdad, in an operation expected to last several days.
The Chicago Tribune report said the offensive "was seeking to uproot a persistent insurgency in an area that American intelligence indicated has become a haven for foreign fighters flowing in from Syria".
Some US forces were north of the Euphrates River, but most were stuck south of the waterway as engineers tried to build a pontoon bridge there.
The report quoted some marines as saying residents of one riverside town had turned off all their lights at night, apparently to warn neighbouring towns of the approaching US troops.
US and Iraqi forces appear to have stepped up raids on suspected insurgent hideouts in recent weeks, including some near the Syrian border, where US officials say foreign militants are crossing into Iraq to attack coalition forces.
On Sunday, the US military said coalition forces killed six insurgents and detained 54 suspects in raids targeting the country's most feared terror group, al-Qaida in Iraq, in Qaim, a Syrian border town about 200 miles west of Baghdad.
The crackdown comes amid a torrent of insurgent violence, which has killed more than 310 people since April 28, when a new Iraqi government was announced with seven positions left undecided. At least nine American servicemen were killed in weekend attacks.
The interim National Assembly on Sunday approved six more Cabinet members, including four more Sunni Arabs. But the Sunni man selected as human rights minister turned down the job on the grounds of tokenism, tarnishing the Shi'ite premier's bid to include the disaffected minority believed to be driving Iraq's deadly insurgency.
The five new members were sworn in yesterday. The rest of the cabinet also repeated the oath of office after new wording was added at the request of Barham Salih, the Kurdish planning and development co-operation minister. The ministers pledged their allegiance to a "federal, democratic" Iraq, and Mr Salih said that this phrase brought the wording of the oath in line with the language in Iraq's transitional law.
Iraq's two main Kurdish factions, which hold 75 seats in the 270-member National Assembly, are pressing for a federal government that would give strong autonomy to the Kurdish north.
When complete, the new government is expected to include 17 Shi'ite ministers, eight Kurds, six Sunnis and a Christian.
Three deputy premiers have been named one each for the Shi'ites, Sunnis and Kurds, with the fourth held open for a woman.
Prime minister Ibrahim al-Jaafari pledged on Sunday to take "all necessary measures" to restore security in Iraq and said the government could impose martial law, if necessary, to fight the insurgents.
Violence continued yesterday with at least three Iraqis killed in a suicide car bombing at police checkpoint at a busy intersection in southern Baghdad.
The dead included two policemen and a civilian.