A British medical worker kidnapped in Afghanistan has been dramatically freed in a special forces raid. The SAS are understood to have carried out the helicopter raid on the cave where Helen Johnston, 28, who was working for an aid project, was being held along with three other hostages. One of the other hostages is Moragwa Oirere, 26, a Kenyan-born aid worker who had previously worked with Save the Children.

The other two were Afghan colleagues the pair had been working with in the country. The four rescued hostages were reported to be in a "good condition".

The operation happened under cover of darkness in the early hours in Badakhshan province, in the north of Afghanistan. Five heavily armed hostage-takers were killed during the rescue, officials in Afghanistan said. The kidnappers, who are believed to have been a criminal group with links to insurgents in Afghanistan, had made a ransom demand in a video.

Mr Cameron said he authorised the rescue attempt on Friday afternoon after becoming increasingly concerned about the safety of Ms Johnston and her colleagues. He said all four hostages were rescued safely, no British troops were injured and a number of Taliban and hostage-takers were killed.

The Prime Minister added the rescue should serve as a warning to terrorists across the world who take British citizens hostage. He said: "They should know if they take British citizens as hostage we do not pay ransoms, we do not trade prisoners. They can expect a swift and brutal end."

The raid in the remote province of Badakhshan came less than two weeks after the women had been seized while trekking on horseback to treat villagers suffering from malnutrition.

SAS soldiers, assisted by other troops from ISAF's Joint Special Forces Group, which includes elements American Delta Force soldiers and Navy Seals, as well as local Afghan security forces, were transported to the cave by heilcopter and stormed into it, freeing the four hostages.

The aid workers – Miss Johnston and Moragwe Oirere and their two Afghan colleagues - were kidnapped on May 22. They worked for Medair, a humanitarian non-governmental organisation based near Lausanne, Switzerland. Medair said its team had been abducted while "visiting relief nutrition, hygiene and health project sites" in Badakhshan province.

Miss Johnston studied at the London School of Hygiene and Tropical Medicine, and had worked for the charity in Afghanistan since last year. Last December she spoke of her work and told how she had regularly seen skeletal and "other-worldly" children in Badakhshan province.

Miss Oirere was born and educated in Kenya and subsequently worked for Save the Children in Africa, as well as other aid projects, before working in Afghanistan.

The deeply conservative area, in which women are unable to go out alone and have been beaten for taking their children for treatment, has one of the highest infant mortality rates in the world.

A statement from the coalition described the kidnappers as Taliban, but local officials said they were petty criminals.