Iraq lost contact with soldiers during an operation Sunday aimed at rescuing a battalion that was repeatedly attacked by militants near the city of Fallujah, the defence ministry said.
"Communication was lost with some of our heroic soldiers... who were injured or wounded during the operations," the ministry said in a statement.
The assault involving army, police, counter-terrorism units and Shiite volunteers was aimed at pushing back Sunni militants who have been besieging an army base for more than a week.
"Forces from the interior and defence ministries, counter-terrorism forces and volunteers are advancing to clear the Al-Sijr area," Iraqi military spokesman Qassem Atta told AFP earlier on Sunday, referring to an area near Fallujah, west of Baghdad.
He said the operation was backed by US air support, without elaborating.
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There was no immediate confirmation from the Pentagon of any US air strikes in the area, which is barely 50 kilometres (30 miles) west of the capital Baghdad.
Atta said the base was located north of Fallujah, a Sunni city which has been out of government control since January, between the militant-held villages of Saqlawiyah and Al-Sijr.
"This operation is aimed at thwarting attempts by the jihadists to seize the battalion headquarters," an army officer involved in the operation told AFP.
"They have been trying for 10 days," he said.
The Islamic State jihadist group led a sweeping offensive in June that overran much of the country's Sunni Arab heartland, and it has a strong presence in Fallujah.