The United States has asked the authorities in Yemen to provide more protection for its citizens after the murders of three American missionaries by a suspected Islamic militant.

Our intention is to bring to justice any and all people who were responsible for these murders White House spokesman

Deploring the attack, a White House spokesman said American and Yemeni investigators were working together to bring those responsible to justice.

A man has been arrested who said he belonged to a local extremist group, Islamic Jihad, and attacked the medical staff because they were Christian missionaries, according to Yemeni police.

American officials say it is too early to tell if the killings are linked to al-Qaeda, which the United States blames for previous attacks in Yemen.

This is the latest in a string of attacks by Islamic militants against Western targets in Yemen, despite the Yemeni Government's co-operation with Washington's war on terror.

Call to Bush

"We strongly condemn and deplore the murder of three American citizens who were providing humanitarian assistance to the Yemeni people," said White House spokesman Scott McClellan.

ATTACK VICTIMS

Martha Myers, 54, from Alabama - killed William Koehn, 60, from Texas - killed Kathleen Gariety, 53, from Wisconsin - killed Donald Caswell, 49, from Texas - seriously injured

"Our intention is to bring to justice any and all people who were responsible for these murders," he said.

Monday's attack, at a Christian-run hospital in the southern town of Jibla, was also condemned by Yemeni President Ali Abdullah Saleh.

"We are confident that such a criminal act won't affect the friendship and cooperation between our countries," he was quoted as telling President George W Bush in a telephone call.

There are about 30,000 US citizens, mainly of Yemeni origin, living in Yemen.

'Closer to Allah'

The attacker walked into the Jibla hospital, about 170 kilometres (105 miles) south of the capital Sanaa, posing as a patient, cradling his gun in his clothes as if it were a baby.

RECENT VIOLENCE

December 2002: Senior Yemeni opposition politician, Jarallah Omar, assassinated November 2002: CIA unmanned plane kills six alleged al-Qaeda members October 2002: Attack on French tanker, the Limburg, blamed on al-Qaeda October 2000: USS Cole attacked in Aden, killing 17 US sailors. Attack blamed on al-Qaeda

He went into a room where staff were holding a morning meeting and opened fire.

Hospital administrator William Koehn, purchasing agent Kathleen Gariety and doctor Martha Myers were killed by shots to the head.

A fourth American, pharmacist Donald Caswell, was also badly injured but is expected to recover.

Donald Caswell was shot in the stomach

Ali Abdulrazzak al-Kamel, who was arrested soon afterwards, said he carried out the attack to "cleanse his religion and get closer to Allah", unnamed officials told the Reuters news agency.

Mr Kamel is a student at Yemen's al-Iman university - which was briefly closed last year after allegations that it was a hotbed of Islamic militancy.

The Christian missionary group to which the victims belonged said the three "martyrs" had been well aware of the risks they were taking.

"Our people are committed to continuing ministering there," said Jerry Rankin, President of the Southern Baptist International Mission Board.

The mission has been working at the Jibla hospital for 35 years.

Al-Qaeda activity

Since the 11 September 2001 attacks, Yemen has been cracking down on suspected al-Qaeda members in an attempt to shed its image of being a haven for Islamic militants.

The US believes al-Qaeda members went into hiding in Yemen after the collapse of the Taleban in Afghanistan last year.

Al-Qaeda has been blamed for two attacks off Yemen's coast - the French tanker Limburg was attacked in October, two years after 17 US sailors died in the bombing of the USS Cole.

Both US and Yemeni forces carry out operations against suspected al-Qaeda members.

Last month, a CIA drone fired a missile which killed six suspected militants in Yemen.