The US military in Iraq says it believes three of its soldiers missing after an ambush have been abducted by militants from, or linked to, al-Qaeda.
US troops are operating in some of their most dangerous areas
Some 4,000 US soldiers are searching for the three men, who disappeared on Saturday after their patrol came under attack south of Baghdad.
The Islamic State of Iraq, an umbrella group of militants led by al-Qaeda, says it is holding the soldiers.
It told the US to stop searching for them if it wanted their safety.
"Your soldiers are in our hands," the group said in a statement posted on an Islamist website. "If you want them safe, do not search for them."
There were no pictures of the soldiers to back up the group's claim.
'Highly credible intelligence'
The attack on the patrol, in the town of Mahmudiya, left four US soldiers and an Iraqi interpreter dead.
"At this time, we believe they (the three missing soldiers) were abducted by terrorists belonging to al-Qaeda or an affiliated group and this assessment is based on highly credible intelligence information," chief military spokesman Major-General William Caldwell said.
US troops are carrying out an intensive search in the area, known as the Triangle of Death because attacks by insurgents are so common.
Officials said operations were focussing on Mahmudiya's outlying towns and villages.
"US and Iraqi forces are cordoning off the farmsteads and orchards in all these areas," Mahmudiya Mayor Moad al-Amiri said.
In the town of town of Yousifiya, US troops broadcast warnings for residents to stay indoors, as units searched house-to-house for the missing men.
Forces on the ground around Yousifiya were being supported by aircraft and satellites.
Gen Caldwell said that the search would continue until the men were found.
Last year, two American soldiers were kidnapped in the same area - their bodies were found several days later.
Elsewhere in Iraq, at least eight people were reported killed in a series of attacks in and around Baghdad.