The BBC's Ben Brown: "Things went tragically wrong"

The UK's Foreign Secretary, Robin Cook, confirmed the deaths on Tuesday night.

Mr Cook said "others" were injured in the incident and all next of kin had been informed.

The BBC's Rageh Omaar: The Yemeni government had had enough

He said any British visitors still in Yemen should leave.

The four dead were among a party of 16 Western tourists - 12 Britons, two Americans and two Australians - who were travelling with the British-based tour operators Explore Worldwide. They were abducted on Monday, despite a police escort.

Brian Hanrahan: "A tougher approach to kidnapping"

Two of the kidnappers are also believed to have been killed by the Yemeni security forces in a shoot-out in the southern province of Abyan.

Four others have been arrested and face the death penalty under legislation introduced in the summer in a bid to stamp out kidnapping in the Yemen.

Yemeni officials say the security forces intervened when the kidnappers - believed to be Islamist militants - began killing their captives.

BBC Correspondent Frank Gardner: "This is a black day for the Yemen"

However, our correspondent says a source close to Yemen's President Ali Saleh told him the casualties were the unfortunate result of a planned operation to free the hostages. He says it appears to have been a massive "botch-up".

He says it appears the Yemeni authorities were unwilling to discuss the kidnappers' demands, which included the release of the leader of an Islamist group, arrested two weeks ago.

If the details are confirmed, this marks a departure from the pattern of peaceful kidnappings of foreigners in Yemen. Westerners have generally been held only briefly in support of local grievances.

Four still held

About 100 foreigners in have been kidnapped in Yemen in the past six years.

Four German tourists abducted three weeks ago are still being held in the eastern province of Marib.

The tourists were travelling with the UK-based company, Explore Worldwide, in conjunction with a Yemeni holiday firm.

Explore Worldwide spokeswoman Sue Ockwell: "All trips to Yemen are on hold"

A spokesman for Explore Worldwide, who are based in Aldershot, Hampshire, said all trips to Yemen were on hold while their operations were "under review".

Unlike Chechnya, Yemen is not on a list of countries which the British Foreign Office advises tourists against visiting on any account.