An explosion killed three British soldiers in Afghanistan's southern Helmand province, a spokesman for NATO-led forces said on Thursday, where U.S. troops are due to be sent to try to turn the tide against Taliban insurgents.

President Barack Obama last week ordered 17,000 more U.S. troops to southern Afghanistan and many of them will be sent to Helmand province, the world's biggest opium producing region, where British troops clashes daily with Taliban insurgents.

The blast happened while the troops were on an escort operation in the Girishk district of Helmand province on Wednesday, the British Defence Ministry said.

A helicopter was called in to evacuate the soldiers, but a doctor pronounced the men dead on the helicopter before they could reach hospital.

Military commanders predict violence will rise further from last year's record levels as the extra U.S. troops enter the south and attempt to clear insurgent strongholds.