A spokesman for Merlin said: "There is strong diplomatic support to help the group reach Guinea [but] this could take some time if the border remains closed."
A three-man team of British diplomats led by Haydon Warren-Gash, the British ambassador in Abidjan, capital of neighbouring Ivory Coast, was travelling to Liberia yesterday to assist the Liberian government in an effort to help free all six of the confirmed Western hostages.
The aid workers were abducted in Kolahun, north west Liberia, early on Wednesday, the latest in a series of kidnapping in west Africa that also have seen Britons seized in Sierra Leone and Nigeria. There has been no indication of what the kidnappers want.
Dr Mike Roe, 33, from London, midwife Sara Nam, 30, from Carmarthen, west Wales, and transport expert David Heed, 26, from Wellingborough, Northamptonshire were all working for Merlin, rehabilitating hospitals and clinics destroyed in Liberia's long civil war.
Also seized 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.
On Thursday night, Merlin medical director Dr Bruce Laurence said the charity had made radio contact with Dr Roe. Mr Laurence said: "They are still under the control of the armed gang but he assured us that they are all well and unharmed."
The United Nations High Commission for Refugees said that the heightened tensions forced 50 aid workers and their dependents to cross the border yesterday into Guinea.