Nato and Afghan forces have begun what they say is their largest offensive against the Taleban in the south.
British troops have met stiff resistance from the Taleban
Operation Achilles will eventually involve more than 4,500 Nato troops and nearly 1,000 Afghan soldiers in Helmand province, the alliance says.
Most of the Nato troops will be British, with US, Dutch and Canadian troops also taking part.
Last month Helmand's governor said up to 700 insurgents had crossed to fight British forces.
The operation began at 5am local time (0030 GMT) and will focus on the northern part of Helmand, said officials.
Shortly after announcing the operation, a Nato statement said one soldier had been killed in combat operations.
It was later confirmed that the soldier was a British marine.
"This is the largest multinational combined ANSF [Afghan National Security Force] and Isaf [International Security Assistance Force] operation launched to date and it signifies the beginning of a planned offensive to bring security to northern Helmand," the head of the alliance's southern command, Dutch Major General Ton van Loon, had said earlier.
It would focus on "improving security in areas where Taleban extremists, narco-traffickers and other elements are trying to destabilise the government of Afghanistan", his statement said.
The operation in Helmand, where thousands of mainly British troops have been fighting daily battles against the Taleban, was launched at the request of the Afghan government, Isaf said.
The rebels have been engaging British forces around the Kajaki dam - a strategically important hydro-electric plant that the military wants to secure so that it can be repaired and put to work.
"Strategically, our goal is to enable the Afghan government to begin the Kajaki project," the statement said.
"This long-term initiative is a huge undertaking and the eventual rehabilitation of the Kajaki multi-purpose dam and power house will improve the water supply for local communities, rehabilitate irrigation systems for farmlands and provide sufficient electrical power for residents, industries and commerce," Maj-Gen Van Loon said.
"Isaf and ANSF forces will continue to apply pressure to extremist forces and pursue reconstruction and development objectives until they are achieved," he added.
Meanwhile, US troops have come under heavy criticism after nine Afghan civilians were reported killed in a bombing raid by US troops in Kapisa province on Sunday.
Reports said hundreds of people protested in the city of Jalalabad, near Pakistan, blocking the highway to Kabul.
On Sunday, US forces were also accused of killing 10 civilians during a shoot-out in Nangarhar province, prompting demonstrations in the area and condemnation from President Karzai.