"Early this morning, 30 Marines and one sailor of the 1st Marine Division and the 3rd Marine Aircraft Wing of the 1st Marine Expeditionary Force died when their CH-53E 'Super Stallion' helicopter crashed near Ar Rutbah [in] Al Anbar Province here in Iraq," said Lt General John Sattler from his headquarters at Falluja.
The US military had earlier issued a statement saying "a US Marine Corps transport helicopter crashed on 26 January at approximately 1.20am (2220 GMT Tuesday) near al-Rutbah while conducting security and stabilisation operations".
Earlier in the day, speaking to Aljazeera from Ramadi, Muhammad Abd al-Rahman, an independent Iraqi journalist, said local residents reported seeing several US military helicopters on low-altitude flights over the area where the crash took place, which is apparently beyond the reach of civilians.
US forces use transport aircraft for ferrying both goods and troops in response to frequent attacks on US convoys on Iraq's roads, Abd al-Rahman said.
But as these aircraft fly at relatively slow speeds and at low altitudes, they are vulnerable to rocket attacks by fighters who can fire and withdraw.
No group has claimed responsibility for the downing of the helicopter so far, he added.
The US Marines declined to provide the cause of the crash.
Four US marines were killed in
the western province of Anbar
It said a search-and-rescue operation was under way. The statement said casualties would be confirmed later.
In a separate incident, four US marines were killed in the western province of Anbar on Wednesday, the military said in a statement.
One soldier was killed in a clash with fighters north of Baghdad, the US military also said, while another was killed in a bomb blast in the capital itself.
"A Task Force Baghdad soldier died from an improvised-explosive-device detonation about 4.30 pm (1330 GMT) 26 January," said a US military statement.
The latest fatalities mark the highest single-day toll of US soldiers in Iraq - 37 - pushing the total to 1418 since the March 2003 invasion.
The crash came as US-led forces, Iraqi national guardsmen, interim government offices and other election targets were hit around the country.