The corpse was found on Wednesday evening near Baiji, 180 km north of Baghdad, said a US military spokeswoman on Thursday.
The remains were being investigated to determine identification, she said.
It is unclear whether the body was that of a Bulgarian hostage killed by his captors earlier this week.
Several foreigners taken captive in Iraq have been shown in videos wearing orange jumpsuits.
Unconfirmed reports said that another headless corpse, also wearing an orange jumpsuit, was found in the Tigris on Friday.
A diplomat at the Bulgarian embassy in Baghdad said he was aware that a body had been discovered, but it was not known whether it was one of the Bulgarians beheaded on 13 July.
The fact the body was headless hindered identification, he said.
In Sofia, Bulgarian Foreign Minister Solomon Passy said the government had asked for an urgent investigation to identify the body. “At this moment we do not have a confirmation whether it is one of our compatriots,” he told Bulgarian television.
“At this moment we do not have a confirmation whether it is one of our compatriots”
Kidnappers linked to al-Qaida ally Abu Musab al-Zarqawi said a week ago they had captured Bulgarian truck drivers Georgi Lazov, 30, and Ivailo Kepov, 32, and would kill them unless US-led forces released Iraqi prisoners.
Earlier this week Aljazeera television said it had received a video tape showing the decapitation of one of the Bulgarians.
The two Bulgarians were transporting cars to the city of Mosul when they disappeared on 27 June. Bulgaria said they were simple workers, not political people.
Bulgaria has held its ground on policy, saying its 470 troops in Iraq will stay as long as they are needed.
A deadline for the threatened execution of a second Bulgarian captive passed a second day without news.
Several foreigners are missing in Iraq, including a Filipino driver and an Egyptian.