Attacks killed five American soldiers and 20 Iraqis on Monday, the deadliest day for US forces in Iraq in more than two years, just months before all of them must withdraw.
The violence raises major doubts over Iraqi security capabilities ahead of a year-end deadline for the US pullout, with Washington pressing Baghdad to decide soon whether or not it wants an extended American military presence.
"Five US service members were killed Monday in central Iraq," said a brief US army statement. The names and details of the deceased are being withheld until next of kin can be informed, it added.
Captain Dan Churchill, a US military spokesman contacted by AFP, declined to give details on how or where the soldiers died.
An Iraqi interior ministry official said at least three rockets struck a base in east Baghdad where US soldiers were present, but could not specify if it was an American or Iraqi installation.
An Iraqi police official, however, said five rockets were fired at an American base on Baghdad's outskirts.
Both officials said the rockets were fired at dawn.
The deaths were the most of American service personnel in a single day since May 11, 2009, when a US soldier was arrested and charged for having opened fire on five of his comrades on a base just outside Baghdad.
They bring to 4,459 the number of US soldiers who have died in Iraq since the US-led invasion of 2003, according to an AFP tally based on data compiled by independent website www.icasualties.org.
The remaining 45,000 US forces are primarily charged with training and equipping their Iraqi counterparts, although they still take part in joint counter-terror operations.
Their bases still come under frequent rocket attack by insurgents.
Signup to our newsletter and follow us on Facebook and Twitter!
Also on Monday, violence in Baghdad and central Iraq killed 20 people, including 12 struck by a car bomb driven by a suicide attacker in Saddam Hussein's hometown of Tikrit, officials said.
The fatalities in Tikrit included military intelligence Colonel Nuri Sabah al-Mashhadani and two other officers, while 20 other people were wounded, according to a police captain and an army captain, both speaking on condition of anonymity.
Nine soldiers were among the dead, including the three officers.
The explosion struck at 9:30 am (0630 GMT), targeting the main gate of a fortified compound housing several of Saddam's presidential palaces, which is home to security offices.
The compound is locally called Tikrit's "Green Zone," alluding to the heavily secured centre of Baghdad where parliament and the US embassy are located.
The unrest came three days after attacks at a Tikrit mosque and a hospital where the victims were being treated killed 24 people.
Friday's violence was the worst in Tikrit since a March 29 Al-Qaeda raid on the city's provincial council offices, which led to an hours-long firefight with security forces that killed 58 people.
Tikrit is the capital of mainly Sunni Arab Salaheddin province, which was a key battleground in the insurgency that followed the US-led invasion.
In the western city of Ramadi, insurgents detonated bombs around the house of a local police chief overnight, killing four of his family members and wounding two others, a police official in the city said.
The police chief, Major Jumaa Abdulrahman Aswad, was not at the house when the blasts struck.
Three separate attacks in Baghdad also left four people dead, including a soldier, and 12 others wounded, security officials said.
Iraqi leaders are still considering whether to request an extension of the US military presence, and top American officials have pressed their Iraqi counterparts to decide soon.