The crew, who included reporter Rageh Omaar, were preparing a special report on kidnappings for the BBC2 programme Correspondent. They were detained by interior ministry troops six days ago.

They were returning to the capital Sana'a from recording footage in the lawless north of the country when they were arrested.

Plea to Yemeni premier

A BBC spokesman expressed "increasing concern" for the welfare of the men as it was disclosed that British ambassador Vic Henderson has made direct representations to the Yemeni Prime Minister, Dr Abdul Iriyani.

Mr Omaar, producer Robin Barnwell and cameraman Frank Smith have had their passports and equipment confiscated and are thought to be under arrest at a hotel in the city.

The country's attorney-general is investigating claims that they were filming without Government approval.

A Foreign Office spokesman said: "We are working round the clock to secure their release.

"The journalists have been visited every day by embassy staff and approaches are being made at the highest levels.

"The ambassador has this weekend met with Prime Minister Iriyani and raised this matter.

"We understand the Yemenis are investigating what the BBC was doing in the area and why they were filming."

The journalists were reported to have been released to the British ambassador on Friday after two days' detention at the interior ministry.

But the corporation insisted the men had merely been allowed to return to their hotel to change their clothes and are still being held under armed guard.

BBC 'concern'

A spokesman for BBC News said: "Our understanding is the three are being held in their hotel and we are increasingly concerned for their well-being.

"They were in the country will full accreditation and made clear what it was they wanted to film.

"We want to see them released and flown back to Britain as soon as possible and taking every step possible to ensure that happens."

The spokesman added that the BBC had been in touch with the families of the three men, who were also "very concerned".