US troops in the Iraqi capital, Baghdad, are building a wall around the Sunni district of Adhamiya, which is surrounded by Shia communities.
The wall is meant to prevent sectarian attacks in Baghdad
The 5km (three-mile) concrete wall is part of a strategy to "break the cycle of sectarian violence", a US military spokesman said.
Adhamiya lies on the mainly Shia Muslim east bank of the Tigris river and has been badly hit by sectarian attacks.
The wall has provoked an angry reaction from residents.
US military spokesman Lt Col Christopher Garver said it was "not the stated goal of the Baghdad security plan to divide everything up into these... small gated communities".
But the BBC's Andrew North, in Baghdad, says troops have already dubbed it "the Great Wall of Adhamiya".
When the wall is complete at the end of the month, say US commanders, residents will only be able to cross the 3.6 metre (12 feet) high wall through several checkpoints guarded by US and Iraqi troops.
Similar walls are being planned for two other areas of Baghdad.
Iraq has been in the grip of raging sectarian violence since the bombing of an important Shia shrine in Samarra in February 2006.