SANAA, Mar 16, 2009 (AFP) - Four South Korean tourists were killed and four more wounded by an explosion in the historic city of Shibam in southeast Yemen on Sunday, a Yemeni security official and Seoul's foreign ministry said.

"According to a preliminary toll, the explosion killed four South Korean tourists and wounded four more, as well as a Yemeni man," the official told AFP by telephone from Shibam in Hadramawt province.

"We think the blast was caused by a bomb that had been planted there," the official added, speaking on condition of anonymity.

The security official said the blast came as the tourists were photographing the sunset and that an inquiry had been launched.

"The explosion happened as they were gathered on a hill called Khazzan that overlooks the city. They were on foot and taking pictures of the buildings in Shibam at the moment the sun went down," he added.

In Seoul a foreign ministry statement confirmed the death of two South Korean men and two women in an apparent bomb attack.

The four wounded are in hospital but not in critical condition, a ministry official told Yonhap news agency. The Seoul government called an emergency meeting of related agencies.

Shibam, some 800 kilometres (500 miles) southeast of Sanaa, is a UNESCO world heritage site well known for its high-rise mud-brick buildings that have given it the nickname of the "Manhattan of the desert."

In January 2008 two Belgian tourists were shot dead along with their local guide and driver in Hadramawt.

Two months later, the US embassy was the target of mortar fire that missed and hit a school, killing two people.

A car bomb attack in Marib, also east of Sanaa, in July 2007 killed eight Spanish holidaymakers and two Yemeni drivers.

That attack took place at the entrance to Mahram Bilqis, an ancient oval-shaped temple that legend says belonged to the Biblical Queen of Sheba.

In January the local Al-Qaeda branch announced in a video message posted on the Internet the merging of the Saudi and Yemeni branches into "Al-Qaeda in the Arabian Peninsula," led by a Yemeni called Nasser al-Wahaishi.

Yemen is the ancestral homeland of Osama bin Laden, and Al-Qaeda has launched many operations there -- notably the 2000 attack on the USS Cole in the port of Aden that killed 17 sailors.

The US embassy in the capital was targeted last September by a double car bomb attack claimed by Al-Qaeda that killed 19 people, including seven attackers.

Some Western embassies are now concealed behind five-metre-high (16-foot) blast walls, and some diplomats have said they believe there is an influx of militants into Yemen.

In April 2008 a complex of villas inhabited by US oil experts in Sanaa was hit by rockets, and the same month the Italian embassy also came under attack.

Oil installations have also been frequent targets in Yemen, which had a modest production of 300,000 barrels a day in 2008.

A security official in Sanaa said on February 8 the authorities had decided to release 176 people suspected of links to Al-Qaeda, without saying why.

Sixteen days later a security court sentenced three members of an alleged Al-Qaeda cell to seven years each in jail.

They were convicted of conspiring to attack tourists and government facilities to avenge the killing of an Al-Qaeda commander in August last year.

Few tourists visit Yemen, which also has a history of abductions of Westerners by powerful tribes who then use them as bargaining chips with the authorities. Those kidnapped are generally freed unharmed.

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Copyright AFP 2009.