Sara Nam, 30, from Carmarthen and five colleagues were abducted at Kolahun, in the north-west of the West African country, near the borders with Sierra Leone and Guinea, between 0700 and 0800 local time on Wednesday.

BBC's Mark Doyle on the steps being taken to contact the kidnappers

A Merlin spokeswoman said it appeared the group of hostages had been kept together.

"There has been radio contact with Dr Mike Roe, one of the three Merlin staff who were abducted, and he was able to confirm that they are now close to the Guinea border," she said.

"We believe it's too late for the aid workers to cross into Guinea and they still appear to be under the control of the armed group which abducted them."

Gunmen on the streets of Liberia

The London-based agency said Dr Roe confirmed the six had not been harmed.

Its medical director, Dr Bruce Laurence, said the charity's next aim was to get in touch with the kidnappers.

The UK Foreign Office says it is doing all it can to secure their release.

A British ambassador and a team of diplomats are being sent to the capital, Monrovia, to help negotiate with the kidnappers.

A spokesman said it was "making clear to the Liberian Government that the safety of the abductees is the paramount consideration".

Volunteers from the British medical charity Merlin

Midwife Miss Nam, Dr Roe, 33, from London and transport expert David Heed, 26, from Wellingborough, Northamptonshire were abducted by an armed gang in Kolahun, north west Liberia, early on Wednesday.

Also seized in the raid on a compound close to the Guinea and Sierra Leone borders were an unnamed Briton working for the New York-based International Rescue Committee and a Norwegian man and an Italian woman working for the charity Medecins Sans Frontieres.

It is the third incident in a spate of recent kidnappings of Britons in west Africa, and the Foreign Office has not ruled out the possibility the abduction was a "copycat" kidnapping following the recent hostage-taking in Sierra Leone.

The Merlin team was helping to rehabilitate hospitals and clinics destroyed in Liberia's long civil war.