One of the greatest wonders of civilisation, and probably the world's most ancient structure - the Sumerian city of Ur in southern Iraq - has been vandalised by American soldiers and airmen, according to aid workers in the area.

They claim that US forces have spray-painted the remains with graffiti and stolen kiln-baked bricks made millennia ago. As a result, the US military has put the archaeological treasure, which dates back 6,000 years, off-limits to its own troops. Any violations will be punishable in military courts.

Land immediately adjacent to Ur has been chosen by the Pentagon for a sprawling airfield and military base. Access is highly selective, screened and subject to military escorts, which - even if agreed - need to be arranged days or weeks in advance and carefully skirt the areas of reported damage.

There has been no official response to the allegations of vandalism - reported to The Observer by aid workers and one concerned US officer.

Ur is believed by many to be the birthplace of the prophet Abraham. It was the religious seat of the civilisation of Sumer at the dawn of the line of dynasties which ruled Mesopotamia starting about 4000 BC. Long before the rise of the Egyptian, Greek or Roman empires, it was here that the wheel was invented and the first mathematical system developed. Here, the first poetry was written, notably the epic Gilganesh, a classic of ancient literature.

The most prominent monument is the best preserved ziggurat - stepped pyramid - in the Arab world, initially built by the Sumerians around 4000 BC and restored by Nebuchadnezzar II in the sixth century BC.

The Pentagon has elected to build its massive and potentially permanent base right alongside the site, so that the view from the peak of the ziggurat - more or less unchanged for 6,000 years - will be radically altered.

Each hour, long convoys of trucks heave gravel and building materials through checkpoints and the barbed wire perimeter extends daily.

There are reports that walls have been damaged by spray-painted graffiti, mostly patriotic or other slogans, and regimental mottos. One graffiti reads: 'SEMPER FE' - Always Faithful - the motto of the Marines, who stormed through this region on their way to Baghdad, and form a contingent at the base.

Other reports by groups who cannot be named for fear of losing access to medical patients being treated on the base say there has been widespread stealing of clay bricks baked to build and restore the structures at Ur.

The Army Public Affairs office at Ur refused to speak to The Observer.