The hunt is continuing with renewed urgency after three female hostages in the group - two German nurses and a South Korean teacher - were found dead on Monday.

The fate of a British national, a German technician, his wife and their three children was still unknown on Tuesday evening.

The government of Yemen has vowed to track down the kidnappers and has offered a 25,000 dollar reward (17,500 euros) for information leading to the capture of the perpetrators.

"The security apparatus will continue to hunt the terrorist group which committed this crime and bring them to justice," said Yemeni Foreign Minister Abubakr al-Qirbi in Sana'a.

In Germany, Chancellor Angela Merkel has condemned the killing of the three women. "We must unfortunately assume that two of the three people found dead in Yemen were German women doing their work," she said. "It is very sad news and we strongly condemn (the killings)."

The three women had worked for the same aid organization in Yemen, the Dutch World Wide Services Foundation, a charity which places medical personnel in hospitals in developing countries.

Police officials in Yemen say the search is concentrating on five districts east of the northern city of Saada near the Saudi Arabian border.

No group has claimed responsibility for the kidnapping and murders, but analysts say the killings bear all the hallmarks of al-Qaeda or related Islamic extremists.

gb/dpa/ap/Reuters

Editor: Susan Houlton