The US military has offered rewards of up to $200,000 (£100,000) for information leading to the return of three of its soldiers missing in Iraq.
Helicopters have dropped leaflets offering rewards for information
Thousands of US and Iraqi soldiers are involved in the search for the men, who vanished after an ambush on Saturday.
The Islamic State of Iraq, an umbrella group of militants led by al-Qaeda, has said it is holding the soldiers.
Four US troops and an Iraqi interpreter were killed in Saturday's attack in the town of Mahmudiya, south of Baghdad.
In an unverified statement posted on an Islamist website, the militant group claiming to hold the soldiers told the US: "If you want them safe, do not search for them."
Some 170,000 leaflets offering the reward of up to 250m Iraqi dinars have been distributed in the Mahmudiya area, the US military said.
Private Daniel Courneya has been named as among those killed
Chief US military spokesman Major General William Caldwell said evidence had been found of a gun battle after an initial roadside bomb attack on two parked Humvee vehicles.
"Obviously it appears there was a fire fight that did ensue and we don't know the exact condition of our three men," he told a news conference.
The Pentagon has named three of the four soldiers killed as Sergeant James Connell, 40, of Tennessee, Private Daniel Courneya, 19, of Michigan and Private Christopher Murphy, 21, of Virginia.
DNA tests are being carried out to determine the identity of the fourth body, which was badly damaged. The military will then be able to confirm which are the missing three soldiers.
The four names in question are Sergeant Anthony Schober, 23, of Nevada, Specialist Alex Jiminez, 24, of Massachusetts, Private Joseph Anzack, 20, of California and Private Byron Fouty, 19, of Michigan.
About 4,000 US troops and 2,000 Iraqi troops have been searching the area where the ambush happened, known as the Triangle of Death because attacks by insurgents are so common, for the past four days.
The US military has said it has "highly credible intelligence information" that the missing soldiers were abducted "by terrorists belonging to al-Qaeda or an affiliated group".
Major General Rick Lynch told the Associated Press news agency that he remained optimistic the three men would be found alive.
"We're pursuing all intelligence," he told AP. "Some of those leads tell us that the soldiers have been taken out of the area but the majority tell us that they're still in the area."
Last year, two American soldiers were kidnapped in the same area - their bodies were found several days later.