Reports say the nuns worked in a psychiatric hospital with the Calcutta-based Missionaries of Charity, the order founded by the late Mother Teresa.

They were shot as they left a clinic in the Red Sea city of Hodeida, about 225 (140 miles) from the capital, San'a.

The reports quote police as saying that their attacker was a mentally ill man who previously had been a patient at the clinic.

The suspected killer was said to have been detained by local residents immediately after the incident and handed over to the police.

Yemeni officials said that President Ali Abdullah Saleh has promised to direct the inquiry into the attack.

Charity in Yemen since 1970

The Hodeida institute is run by 12 nuns and it cares for some 100 elderly or handicapped people.

The Missionaires of Charity have been in Yemen since 1970, working in the main cities.

Officials say its workers have never previously been attacked.

The order now comprises nearly 2,500 nuns around the world, backed by 400 brothers and thousands of lay volunteers who run 380 hospices, leper colonies, orphanages and homes for AIDS sufferers worldwide.