An enlarged UK deployment to Afghanistan has begun with the arrival of 150 Royal Marines Commandos.
The marines will be supporting development of the region
The 42 Commando troops are flying to Helmand, an area of major Taleban activity and opium production in the south, via the capital, Kabul.
The Devon-based troops will provide security for the RAF as it ferries in further UK troops.
The UK Ministry of Defence announced last month that an extra 3,300 troops would be sent to southern Afghanistan.
Their focus is due to be reconstruction rather than counter-terrorism.
The 42 Commando troops will be the first combat troops of the new British deployment, part of an expanded Nato force in southern Afghanistan.
The Nato force is replacing a much smaller American force which is hunting the Taleban.
At Kabul on Wednesday, the commandos transferred from an RAF Tristar aircraft to a Hercules C-130 transport plane for the next part of their journey.
Lt Col Ged Salzano, commanding officer of 42 Commando, said the deployment was "typical of the expeditionary nature of Royal Marines business".
As well as providing security as the Royal Air Force (RAF) ferries in the rest of the troops - from 16 Air Assault Brigade - the 42 Commando troops will also protect engineers setting up a base in Helmand's provincial capital Lashkar Gar.
BBC defence correspondent Paul Wood says the Taleban have promised a wave of suicide bombings against Nato and British troops.
Senior British commanders say the large "footprint" left on the ground by the impact of 3,300 new troops arriving will probably attract an increase in attacks, at least initially.
Lt Col Salzano added: "The company group is robust enough to look after itself and protect the engineers.
"Conditions are expected to be tough but 42 Commando are well prepared for the task."
At the start of the week, Chinook helicopters were unloaded at Kandahar and reassembled to transport 42 Commando troops.
The helicopters will cover vast expanses of desert between towns.
"They're also vital because of roadside bombs," Paul Wood said.
A central aim of the British mission is also to provide security for Afghan forces to eradicate opium farming and trafficking in southern Afghanistan.
Helmand accounts for a significant amount of the opium grown in Afghanistan.
The UK takes control of Nato forces in Afghanistan in May.
As well as the main deployment of 3,300, a further 850 will conduct preparation work for that deployment and about 1,000 troops are to be sent to the Headquarters Group of the Allied Rapid Reaction Corps.
Defence Secretary John Reid has said the total number of UK troops in Afghanistan at any time will not top 5,700.
Nato's International Security Assistance Force (Isaf) mission currently numbers about 9,200 troops. It is expected to increase the overall number to about 15,000.