Seventy walls are in place around the world to protect national borders, compared with a dozen in 1989 when the Berlin Wall came down, according to researcher Elisabeth Vallet of Quebec University.
As US President Donald Trump moves to build a wall along the Mexican border, here are other significant frontier barriers around the globe:
- US: extending existing barrier -
One third of the 3,200-kilometre (2,000-mile) US border with Mexico is, in fact, already protected by a fence. Authorised by US President George W. Bush and completed in 2010, the metallic barrier is more than five metres (16 feet) high and equipped with floodlights and surveillance cameras.
- European barriers against migrants -
- FRANCE: With existing high wire fences not enough to stop them, a one-kilometre "anti-intrusion" wall was built in late 2016 to block access to the northern port of Calais for migrants trying to clandestinely cross the Channel to Britain.
- HUNGARY: As Europe grappled with its worst migrant crisis since World War II, Hungary built a 175-kilometre, four-metre high fence along its border with Serbia in September 2016, followed in October by one along its border with Croatia.
Other countries in the region have also put up barriers to thwart migrants, including SLOVENIA with Croatia, AUSTRIA with Slovenia and MACEDONIA with Greece.
- BULGARIA: Bulgaria in 2014 started building a razor wire barrier along part of its border with Turkey, the main entry point for migrants seeking to avoid the perilous sea route to the EU across the Mediterranean. The barrier is now 176 kilometres long.
- GREECE: Greece became the leading entry point for migrants into Europe at just the moment when its economy was collapsing. So in 2012 Athens built an 11-kilometre barbed wire fence along its river border with Turkey.
- SPAIN-MOROCCO: The Spanish enclaves of Ceuta and Melilla on the North African coast, Africa's only land borders with Europe, are protected by high-tech border fences, each around 10 kilometres long.
- Walls and conflict -
- ISRAEL-WEST BANK: Israel began building its 712-kilometre security barrier edging the West Bank in 2002, saying it would stop attacks by Palestinian insurgents. Critics say it has been used to seize land and establish a de facto border in breach of international law. Nine metres high at some points, the wall -- which is two-thirds finished -- includes watchtowers and electronic surveillance.
- SAUDI ARABIA-IRAQ: Responding to the rise of the Islamic State group, the Saudis in 2014 added to an existing seven-metre-high sand embankment on the Iraqi border with a 900-kilometre fence and electronic surveillance system.
- INDIA-PAKISTAN: India built a barrier almost 750 kilometres long, along the de facto border dividing disputed Kashmir with Pakistan, to keep out Pakistani militants.
It has also surrounded Bangladesh with a 2,700-kilometre barbed-wire fence aimed at restricting immigration and smuggling.
- Marking territory -
- NORTH AND SOUTH KOREA: The most heavily militarised border in the world is the one drawn between North and South Korea in 1953. Bristling with razor wire, sensors, landmines and heavy weapons, the Demilitarised Zone stretches for 250 kilometres along the full length of the border.
- WESTERN SAHARA: A 2,700-kilometre sand wall, in place since the 1980s, has been built between the 80 percent of territory under Moroccan control and zones under Polisario rebels, who have been fighting and negotiating for control of Western Sahara since the 1970s.
- CYPRUS: A wall splits the island and its capital Nicosia between its Turkish and Greek Cypriot halves, dating back to Turkey's invasion in 1974.