Five US soldiers have been killed in central Iraq, the US military has said.
Iraqi security sources said there had been a rocket attack, with other reports saying the target was on the outskirts of Baghdad.
The BBC's Jim Muir says the US military has suffered its single most serious incident in Iraq in over two years.
Elsewhere in the country on Monday, at least 21 people were killed in twin bomb attacks, one on a mosque, in the northern city of Tikrit.
US fatalities in Iraq have been rare since Washington officially ended combat operations in the country last August, leaving about 45,000 troops.
The role of the US army, which is due to pull out of Iraq on 31 December, is to advise and help the security forces.
The military statement simply read: "Five US service members were killed Monday in central Iraq."
The names of the deceased are being withheld until next of kin have been informed.
Agence France-Presse news agency quoted an interior ministry official and an Iraqi police officer as saying five rockets had struck the Camp Victory base on the edge of Baghdad.
The Iraqi officials also told AFP that two apparent insurgents were found dead outside the base, apparently killed by their own rockets exploding in their vehicle. Neither report has been confirmed.
However, Associated Press news agency said the Americans were killed in their living quarters on an Iraqi base, where they had been serving as advisers.
The independent icasualties count of US service member fatalities in Iraq now stands at 4,459.
A large number of the 45,000 US troops in Iraq are due to leave in the summer.
However, during a visit in April, US Defence Secretary Robert Gates said American troops could, if required by Iraq, stay in the country beyond the withdrawal date of the year end.
Violence has reduced in Iraq over the past few years, but there are still regular attacks and bombings.
In Tikrit on Monday, a bomb exploded outside a mosque during Friday prayers, killing 16 people.
Hours later, a suicide bomber attacked the hospital where the injured had been taken, killing another five.
Our correspondent says Monday's three attacks will again raise questions about how the Iraqi forces will cope once the Americans are gone.