Yemeni authorities have arrested 30 people suspected of belonging to al-Qaeda following an attack on the heavily fortified US embassy in Sanaa, a security source said on Thursday.

Islamic militants launched two car bomb attacks on the embassy on Wednesday, killing at least 16 people, including six attackers. The dead were all Yemeni apart from an Indian woman who was walking past when the attack happened.

”The security authorities want to investigate whether the suspects are linked to Wednesday’s attack,” the security source said.

A US embassy spokesman told Reuters Washington would send a number of teams to Yemen to help the authorities with their investigations.

“This attack is a reminder of the continuing threat we face from violent extremists both at home and abroad,” said the White House. The US would “continue to work with the government of Yemen to increase our counter-terrorism activities”.

The Yemen government, an ally of Washington in its war on terrorism, has been under pressure to do more to tackle the threat of Islamic extremism since the 2000 attack on the USS Cole that killed 17 American sailors. But Yemeni authorities face threats on several fronts as they battle rebels in the north and tribal violence, and are overstretched, say analysts.

The embassy attack was the latest in a string of incidents blamed on Islamic extremists in the unstable country, but it was thought to be the first car bomb attack in Sana’a, the capital.

“For people in Sana’a this is very disturbing, especially for diplomats,” said Heinrich Matthee, an analyst at Control Risks.

A group calling itself Islamic Jihad in Yemen claimed responsibility for the bombing and warned it would carry out attacks on other embassies including those of the UK, Saudi Arabia, and the United Arab Emirates. It is demanding the government release several of its members from jail.

Mr Matthee said there appeared to be at least two extremist cells operating in the country. The more sophisticated one had indirect links to al-Qaeda, and was thought to be responsible for suicide attacks on oil facilities near Mukalla and Marib in September 2006, as well as a suicide attack at a historical site in the north-eastern province last year that killed eight Spanish tourists and two Yemenis.

Attacks this year have included an ambush on a tourist convoy in January that killed twoBelgians, failed mortar assaults on the US and Italian embassies in March and attacks on compounds housing foreign oil workers in April.

Yemen, which is the ancestral home of Osama bin Laden, is the Middle East’s poorest country and has been plagued by instability and rebellions for years, with the government unable to extend its authority across the whole of the tribal society.

Attacks by Islamic extremists have increased since 2006 when militant groups were bolstered by the escape of 23 prisoners – described by the US as known affiliates of al-Qaeda – from a high security prison in Sana’a. Among the escapees was a man known as Nasir al-Wahayshi, who is suspected of having links to al-Qaeda and being a leader of the Yemeni militant cell.

“The security forces are overstretched fighting rebels in the north. There has also been an increase in tribal violence and generally the forces are not doing well, they are under quite a lot of pressure,” said Mr Matthee. “During the next two years there will remain a very serious risk and if the security forces are not able to assure people they can secure the situation, expatriates will become very jittery.”

Oil exports account for about a third of Yemen’s gross domestic product, and it exported about 240,000 barrels a day in 2006. But its reserves are declining and the World Bank describes the nation as the “single largest development challenge in the Middle East”, with limited resources, particularly water.

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