BBC Correspondent Paul Royall: Survivors support allegations of a botched rescue operation

The survivors have said the gun battle, which left four captives dead, was started by the Yemeni security forces and not their kidnappers.

They say the kidnappers only turned on them after government troops attacked and that some hostages were used as human shields.

The BBC's Frank Gardner: The kidnappers said they were prepared to die

But Yemeni Prime Minister Abdel Karim al-Iryani insisted the police assault came after the kidnappers began killing.

"The police were forced to intervene after the abductors began killing their hostages," his condolence message reads.

Survivor Brian Smith: "Abductors used us as human shields"

"There would not have been anyone left if the decision to intervene had not been taken quickly because the terrorist group had not provided any opportunity for dialogue."

The government "did everything to fight terrorism and save the hostages' lives" added the message - expected to be relayed to Downing Street on Thursday.

'Troops fired first'

The BBC's Bob Sinkinson: "Yemeni authorities have expressed their shock"

Yemen is coming under increasing pressure for a full explanation of how its operation to free the 16 hostages went so tragically wrong.

A UK Foreign Office minister, Elizabeth Symons, said Britain would be very concerned if the deaths were the result of a botched rescue attempt.

Survivor David Holmes told the news agency AFP the hostages who died were killed during the security forces' assault on their hideout.

He said he saw a Briton and an Australian shot dead in an exchange of fire with Yemeni troops.

Reports "that the bandits had turned on the hostages, was not the sequence of events,'' he added.

Survivors also reported that government troops fired indiscriminately.

Two other hostages - one American and one British woman - were seriously injured, and three kidnappers killed during the rescue operation.

'Islamic Army' statement

BBC Diplomatic Correspondent Barnaby Mason: British government does not accept Yemeni account of events

Western diplomats are now travelling to the scene of the shoot-out in the southern province of Abyan to establish exactly what happened.

Yemeni officials say three kidnappers were arrested and face the death penalty.

They said the kidnap gang was from an obscure Islamic group calling for the release of their jailed leader.

Middle East expert Hazhir Teimourian: Ramshackle army

A statement purporting to come from the Islamic Army of Aden said the security forces had attacked without warning, because the government could not tolerate demands for an end to what British and American "aggression" against Iraq.

There had been earlier reports that this was one motive for the kidnapping, but British officials say they have no evidence it was linked with the air strikes.

The dead were identified as Ruth Williamson, Margaret Whitehouse and Peter Rowe from Britain and Andrew Thirsk from Australia.

They were among 16 tourists - 12 Britons, two Americans and two Australians - travelling with British-based tour operators Explore Worldwide. They were abducted despite having a police escort.

Foreigners warned

In a separate development, four German hostages abducted in Yemen three weeks ago have been freed.

Britain, Germany and Australia have advised their citizens visiting Yemen to leave as soon as possible.

About 100 foreigners have been kidnapped there in the past six years, but this is the first time hostages have been killed.