A proposal by the United States to build physical barriers between warring communities in Iraq has highlighted the growing number of walls and fences that have been springing up around the world.
Some notable examples:
The most heavily militarised border in the world is that drawn between North and South Korea in 1953. Bristling with razor wire, sensors, landmines and heavy weapons, the Demilitarized Zone stretches for 250 kilometres along the full length of the border.
In June 2002, Israel began building a 650-kilometre barrier edging the West Bank. It includes watch towers and electronic surveillance, and Israel says it is aimed at preventing infiltration by Palestinian guerrillas.
India has built a security fence stretching for almost 1000 kilometres along the "line of control" which divides the contested region of Kashmir between it and Pakistan.
In October last year, China began building a barbed-wire fence along part of its border with much poorer North Korea. Chinese officials played down the move, saying it was part of normal border precautions.
In the same month, US President George W. Bush authorised the construction of a fence along a third of his country's 3200 kilometre border with Mexico, aimed at deterring illegal immigrants. Some 100 kilometres of metal walls are already in place at strategic points.
Also in October 2006, Saudi Arabia announced plans to erect a security barrier along its 900-kilometre border with Iraq. The fence, to include thermal imaging devices, was aimed at preventing the passage of terrorists, a Saudi interior ministry official said.
In 2004, reports emerged that Saudi Arabia was building a high-tech barrier along part of its 1800-kilometre border with Yemen. The Saudi government said the obstacle was made of sand, and was aimed at stopping smugglers.
Spain has built a chain-link fence to prevent would-be emigrants to Europe from entering its tiny territory of Melilla on Morocco's north African coast. In September 2005, Spain said it was doubling the height of the structure.
A 300-kilometre UN-patrolled buffer zone has divided Greek and Turkish Cypriots since 1974. A wall built along part of the line in the centre of the capital Nicosia was dismantled in March this year.
Iron, brick and steel separation "peace lines" still divide some Catholic and Protestant neighbourhoods in Belfast, Northern Ireland. Many of the barriers have remained in place despite recent moves towards peace.
Great walls of the past:
For almost three decades the Cold War border between East and West Germany was materialised by the Berlin Wall, a militarised barrier some 160 kilometres long that entirely separated the enclave of West Berlin from the rest of the city and from East Germany as a whole. The wall was built in 1961 and torn down in 1989, in a move that heralded the collapse of Soviet-dominated communist regimes.
The world's most famous barrier is the Great Wall of China. Work on the 2700-kilometre defensive structure started in the 3rd century BC; today it serves only as a tourist attraction.