A contingent of British forces has taken over security duties in one of the most dangerous Afghan provinces.
Commanders say they will make a difference to ordinary Afghans
The soldiers have replaced US forces in the Taleban-dominated southern province of Helmand, as part of an expansion of peacekeeping operations by Nato.
Hundreds of British troops are already in Helmand and the full complement will eventually number more than 3,000.
The handover comes as US-led coalition forces said they had killed "15-20 enemy fighters" in Helmand on Saturday.
Control was transferred in a ceremony at a base in the province capital, Lashkar Gah, where a union flag replaced the US flag flying over the site.
Elsewhere in Afghanistan, two suicide bomb attacks were reported on Monday.
The attacker was killed - and a passer-by and a foreign soldier injured - in one blast near a US-led coalition convoy in the town of Tirin Kot in southern Uruzgan province, police said.
In eastern Khost province, a man died when explosives strapped to his body went off prematurely, security officials told the BBC.
Two other would-be suicide bombers were hurt but escaped in the incident west of Khost city, officials said.
Violence has been risen sharply in recent months, with a series of roadside bombings against security forces and suicide attacks, mostly in southern and eastern Afghanistan.
Brigadier Ed Butler, commander of UK forces in the country, said he was well equipped for a "challenging mission".
The troops are taking command of Helmand's Provincial Reconstruction Team - one of more than 20 such teams charged with stabilising the country outside the capital, Kabul.
Last week Taleban fighters told the BBC they planned to target and kill British troops in Helmand, one commander labelling the British "an old enemy of Afghanistan".
But Brig Butler said the problems his troops face were "more fundamental" than Taleban or al-Qaeda insurgents.
"We're starting to understand that the nature of the problem is... about tribal issues, it's about water and land rights, it's about feudal and historical matters."
He said British troops had a "very clear" mission and would not go "looking for trouble".
He said Britain had made a "long-term commitment" to the country, and he was determined to "make a difference" to the lives of ordinary Afghans.
The Conservatives have called for clarity over Britain's deployment, saying the government has given "confusing" statements on the nature of the mission.
UK troops will mostly aid reconstruction efforts in Helmand
Shadow Defence Secretary Liam Fox said he wanted to know if the soldiers would be countering insurgents, or "are they going to be there purely for reconstruction, are they going to be anti-narcotics?"
Defence Secretary John Reid denied there was any confusion, saying the mission was "to protect the reconstruction and development of the Afghan economy, democratic government and security forces".
He added: "However, it will be necessary to protect that development against terrorists who seek to destroy all three of those elements, or to attack British troops."
Monday's handover of power in Helmand was described by the Ministry of Defence as of "symbolic significance" because Britain does not yet have its full task force in the province.
The army's 3,300-strong deployment to Helmand, led by 16 Air Assault Brigade, will be completed by June.
There are already about 2,000 British soldiers in Afghanistan as part of the Nato-led International Security Assistance Force (Isaf) and US-led coalition forces.