Nine American soldiers from the same command have been killed in two bomb attacks in provinces north of the Iraqi capital, Baghdad, the US military says.
Six US troops died on Monday while on combat operations in Salahuddin province, in one of the deadliest attacks on American forces in months.
Three other soldiers died in a blast in Diyala province, US officials said.
At least 12 Shia pilgrims were among Iraqis killed in the latest spate of shootings and bombings.
Troops belonging to Task Force Lightning were conducting operations in Salahuddin province when a bomb exploded near their vehicle, killing six soldiers, the US military said on Tuesday.
Thousands of Shia pilgrims are heading towards Karbala
Three others were injured and were taken to a military hospital.
A similar roadside blast in Diyala province killed three Task Force Lightning soldiers and injured one.
Such attacks on American patrols and convoys are a daily occurrence but such a high number of fatalities is unusual, says the BBC's Jim Muir in Baghdad.
In recent weeks American commanders have been openly accusing Iran of allowing the technology for deadly armour piercing bombs to cross the border into that area of Iraq, a charge Iran has denied.
On Saturday, Iranian and American envoys are set to take part in a security conference in Baghdad, involving Iraq's neighbours and other interested parties, aimed at winning support for efforts to stabilise the country.
Another country often accused by the Americans of not doing enough to help, Syria, will also be taking part.
The Task Force Lightning troops were killed on the same day as a car bomb ripped through Baghdad's historic booksellers' market, killing 30 people and injuring 65 others - the first major attack in the Iraqi capital for several days.
The booksellers' district was home to both Sunni and Shia businesses
Baghdad saw further violence on Tuesday when at least seven pilgrims were killed and 14 injured in car bombings as they set off on a journey to Karbala for the religious festival of Arbaeen.
Five other pilgrims were shot dead in Latifiya, 30km (20 miles) south of Baghdad, police said.
There were also reports that five policemen had died in a bombing in a south-eastern suburb of Baghdad on Tuesday, while in central Baghdad a blast killed two civilians, security officials said.
Iraqi and US security forces are in the third week of a major operation to try to stem the violence in Baghdad.
Meanwhile, Iraqi officials have told French news agency AFP that at least 29 suspected al-Qaeda members have been rounded up since Sunday in northern Iraq.
Those detained are said to include two brothers of Abu Omar al-Baghdad, who heads a group known as the Islamic State of Iraqi.
The group last week murdered 14 policemen in revenge for the alleged rape of a Sunni woman and posted a video of the killings on the internet.