PARIS - There are 65 border walls either planned or erected in the world, up from just 16 when the Berlin Wall came down in 1989, according to University of Montreal expert Elisabeth Vallet.
Experts say they serve a primarily political and symbolic function, but they have become increasingly popular with governments as they try to look tough on migration and security.
Here are some of the most recent and significant border walls from around the world:
- Hungary: The most recent wall is the work of Hungary's right-wing government, which began building a 110-mile (177-kilometre) fence along its border with Serbia in July after receiving a record 80,000 asylum applications this year.
- Moroccan-occupied Western Sahara: The oldest functioning security barrier in the world, this 1,700-mile sand wall (or "berm") separates the Moroccan territory from land controlled by Polisario rebels, who have been fighting and negotiating for control of the Western Sahara since the 1970s.
- Saudi Arabia-Iraq: Responding to the rise of the Islamic State group last year, the Saudis are supplementing the existing seven-metre-high (23 feet) sand berm on the Iraqi border with a 560-mile fence, 78 watchtowers, eight command centres, 10 mobile surveillance vehicles, 32 rapid-response centres, and three rapid intervention squads.
- Israel-West Bank: Israel began building its security barrier in 2002, saying it would stop attacks by Palestinian insurgents, though critics say it has been used to seize land and establish a de facto border in breach of international law.
- US-Mexico: Then US president Bill Clinton launched concerted efforts to tighten the border in the 1990s. Fears that the porous border could be exploited by Al-Qaeda - and now the Islamic State group - have led to more and tougher barriers, forced to the top of the political agenda in recent weeks by tub-thumping presidential hopeful Donald Trump, who said in June that he would make Mexico pay to have a wall built along the border.
-Greece-Turkey: A side-effect of improving relations with Turkey and the removal of landmines along their border was that Greece became the leading entry point for migrants into Europe at just the moment when its economy was collapsing. So in 2012 it built the seven-mile Evros wall along the boundary.
- Northern Ireland: Belfast has 99 "peace lines" designed to separate working class Protestant and Catholic communities, the earliest dating back to 1969. They are focused on the north of the city, known as "murder mile" during the period of The Troubles, which lasted until the 1998's Good Friday Agreement. Despite the ceasefire, the walls have only increased in size and number since.
- Spain-Morocco: The Spanish-owned enclaves of Ceuta and Melilla on the North African coast are protected by high-tech border fences. Many have died trying to storm the fences, some shot by Moroccan forces.
- India-Bangladesh: Since 1993, India has been gradually surrounding Bangladesh with a barbed-wire fence aimed at restricting immigration, leading to drawn-out arguments over the precise line of the frontier and leaving up to 100,000 people abandoned in a no-man's land with no public services.
- Cyprus: A wall continues to split the island and its capital Nicosia between its Turkish and Greek Cypriot halves, dating back to Turkey's invasion in 1974.