Up to 72 people were also wounded in the attack on Tuesday afternoon, which targeted a canteen used by soldiers in Forward Operating Base Marez, southwest from the city of Mosul.
Fifty-one were US military personnel, a US military spokesperson said.
In an unverified website statement, an Iraqi group – Ansar al-Sunna – said it carried out the multiple rocket and mortar attacks.
Aljazeera learned that 19 troops were among the dead which included three military personnel whose nationalities were unspecified.
A defence official at the Pentagon confirmed the toll.
However, the US military has been unable to verify whether a missile or mortar shells had struck the camp.
Other reports indicate that the dead troops were all US military personnel. US authorities said the four others killed were not US citizens, but did not give their nationalities.
Another report indicates that the dead included 20 US nationals – 15 servicemembers and five civilian contractors. Two Iraqi soldiers were also killed.
The US military in Iraq issued a statement which verified the time of the noon strike, when the canteen would have been at its most crowded, and said an investigation was under way.
US forces’ casualty figures
Witnesses said two or three explosions hit the camp – the biggest base in the region – and that they saw smoke rising from around the airfield.
Major-General Carter Ham, the commander of the 8000 US troops based in Mosul, said the dead included US soldiers, US and foreign contractors and members of the Iraqi army.
“More than 20 have been killed and more than 60 people have been wounded,” he said, adding that there was a single explosion.
Iraqi town bombed
The raid came as US warplanes launched air strikes on the Iraqi town of Hiyt, west of the capital, killing six Iraqi civilians and wounding nine others.
Children are often victims of US
Hospital officials said a woman and child were among those wounded in the strikes which began on Monday night on the Jamaiya and al-Sinai districts on the eastern edges of the town, which lies about 170km west of Baghdad.
Aljazeera learned that houses, shops and vehicles belonging to civilians were destroyed in the bombing that continued until Tuesday morning.
US marines based in the area had no immediate comment.
Hiyt is situated in the tense Anbar province, which includes Falluja, where US-led forces launched a major offensive last month to target fighters opposed to the US-led interim government.
More troops wounded
In northern Iraq, a roadside bombing wounded five US soldiers and an Iraqi civilian, the US military said on Tuesday.
The bombing near Hawija, west of the northern oil city of Kirkuk, occurred at 11.15am (0815 GMT) on Monday, the military said in a statement.
The six were travelling in a vehicle that was hit by the bomb.
They were listed as being in stable condition. One suspected bomber was detained, the statement added.
Meanwhile, four suspected anti-US fighters were arrested in Awja, the hometown of ousted president Saddam Hussein, just south of Tikrit on Monday night, the military said.