NATO forces rescued two aid workers and their Afghan colleagues in a dramatic pre-dawn raid Saturday. They had been held by militants in a cave in northern Afghanistan for nearly two weeks.

Helen Johnston, a 28-year-old British aid worker, along with Moragwe Oirere, a Kenyan, and the two Afghans were kidnapped on May 22 in Badakhshan province. The four work for a Swiss-based humanitarian non-governmental agency.

The raid, approved on Friday by British Prime Minister David Cameron, was carried out by British troops in co-operation with other NATO and Afghan forces, Cameron told reporters outside 10 Downing Street in London.

The troops swept in by helicopter to free the prisoners from a mountainous and forested area near the Tajikistan border.

"The most important message is to terrorists around the world. They should know if they take British citizens as hostages we do not pay ransoms, we do not trade prisoners. They can expect a swift and brutal end," Cameron said. "It's important that that message goes out."

All four hostages were rescued safely and no British troops were injured. However, a number of Taliban militants and kidnappers were killed, Cameron said.

Afghan officials later said seven militants were killed during Saturday's operation, which was launched around 1 a.m. local time.

Lt. Col. Jimmie Cummings, a coalition spokesman, said a helicopter rescue team reached the scene before dawn and confirmed that the hostages were there.

"The kidnappers were armed with heavy machine-guns, rocket-propelled grenades and AK-47s," Cummings told reporters. "They were kidnapped by an armed terrorist group with ties to the Taliban."

The workers all appeared to be in good health, but were expected to be evaluated before being reunited with their families.

"We are delighted and hugely relieved by the wonderful news that Helen and all her colleagues have been freed," Johnston's family said in a statement.

"We are deeply grateful to everyone involved in her rescue, to those who worked tirelessly on her behalf and to family and friends for their love, prayers and support over the last 12 days."

With files from The Associated Press