GENEVA (AFP) - South-east Asia has become the world's hotspot for pirate attacks after an international clampdown slashed the number of hijackings off the coast of war-torn Somalia, the United Nations said Thursday.

Last year, 28 boats were attacked in the western Indian Ocean but none taken captive in the region, the United Nations Institute for Training and Research (Unitar) said in a report.

That compares to January 2011, when Somali pirates held 736 hostages and 32 boats, some onshore and others on their vessels.

"There has been a significant reduction in the number of pirate attacks during 2013, to the extent one can claim they have almost stopped," Unitar said after its five-year study.

Pirate attacks in the Horn of Africa spiralled from the early 2000s, with pirates hijacking cargo ships and taking crew prisoners for months and even years.

Much of that was down to the international fleet that has started to patrol the Gulf of Aden and Indian Ocean, and many merchant vessels have started keeping armed guards onboard.

Attacks have also become far less severe, with incidents involving rocket-propelled grenades falling from 43 in 2011 to just three last year.

At the same time the pirates' ransom haul fell from US$150 million (S$187.5 million) in 2011 to US$60 million the following year.

They're also sticking much closer to shore, with the average raid taking place less than 50km from the coast in 2013, a quarter of their range three years earlier.

Meanwhile in South-east Asia, piracy has surged, particularly in the maritime trading hub of the Malacca Straits, between Malaysia and Indonesia.

Attacks in the region topped 150 last year after starting an upward trend since 2010, said Unitar.

"Piracy in the Malacca Strait continues to be a major disruptor for safe routes in the eastern Indian Ocean," said the agency.

Last month the International Maritime Bureau (IMB) said that there had been 23 actual or attempted attacks in South-east Asian waters between January and March, mainly off Indonesia.

Unitar said piracy was likely to become even worse in the region as the centre of gravity of global shipping continues to shift towards Asia.

"With changing climatic conditions at high latitudes and medium-to-low-income countries in Asia experiencing the largest growth per capita, additional transport routes may be explored," it added.

Another focus of piracy is Africa's Gulf of Guinea off West Africa, where there were 50 pirate attacks last year.

Attacks in the region have become increasingly violent, sowing economic havoc and sparking a military fight-back by governments.

Early in June a Greek oil tanker with 24 crew aboard was hijacked off Ghana in the Gulf of Guinea, the IMB said.

"The number of attacks show no sign of decreasing. Attacks in the high seas have increased, while attacks in ports are on the decrease," said Unitar.