A homicide bomber on Wednesday struck a convoy carrying South Korean officials sent to Yemen to investigate a bombing earlier in the week that killed four tourists from the country.

The well-coordinated attack was specifically designed to target the South Koreans, officials said. The three-car convoy was carrying South Korean officials and investigators sent to look into a homicide bombing on Sunday that killed four South Korean tourists and their Yemeni driver at a historical site. No one was hurt, officials said.

The South Korean Foreign Ministry confirmed Wednesday's attack and said the convoy was also carrying relatives of the victims of Sunday's attack. They were in the country to recover the bodies of those killed in that attack and were headed to the airport for a flight to return to South Korea when the bomber struck.

Yemen, an impoverished country in the southwestern corner of the Arabian Peninsula, is Osama bin Laden's ancestral homeland and has long been a center of militant activity.

A Yemeni security official said South Korea's ambassador to the country was also in the convoy hit Wednesday, but in Seoul, the Foreign Ministry denied that.

The bomber walked into the road between two of the vehicles and blew himself up as the convoy was traveling to the airport outside the capital, San'a, the Yemeni official said. He spoke on condition of anonymity because he is not authorized to speak to the press. The blast shattered the windows of the vehicles.

Yemeni authorities said Al Qaeda was behind Sunday's homicide bombing near the ancient fortress city of Shibam and that 12 suspects were arrested.

After that attack, South Korea instructed its overseas diplomatic missions to strengthen security measures and said it would set up a body to consult with Middle East countries and other nations prone to attacks to exchange information on terrorism.

The South Korean ambassador met later Wednesday with Yemen's foreign and interior ministers.

State Department spokesman Robert Wood said during a briefing in Washington that the U.S. was concerned about the situation in Yemen and called the two attacks "very troubling."

The U.S. Embassy in Yemen on Wednesday said in a message on its Web site that Embassy personnel had been advised to limit their travel and warned American citizens in the country to exercise caution in the wake of the attack.

In June, South Korea will begin importing natural gas from Yemen. But Yemen's poor security has limited trade between the two nations — it totaled $246 million in 2008. The South Korean government has also sent $4 million in aid to Yemen since 1991, according to the South Korean Foreign Ministry.

Efforts by Yemen's government have failed to root out Al Qaeda and other Islamic militants.

Militants have carried out several deadly attacks on tourists, foreign diplomats, the U.S. Embassy, other Western targets and military installations in the country.

In January 2008, suspected Al Qaeda militants opened fire on a convoy of tourists in Hadramut, killing two Belgians and their Yemeni driver. A homicide bomber detonated his car among tourists at an ancient temple in central Yemen in July 2007, killing eight Spaniards and two Yemenis.

Yemen was also the site of the 2000 USS Cole bombing that killed 17 American sailors.