Nato forces in Afghanistan are ready to take on the Taleban, the peacekeepers' new British commander has said.
Nato plans to boost forces in southern Afghanistan
They will be dealt with "wherever they may be", Lt Gen David Richards said as the UK took over the force from Italy.
The new role is the latest step which will put Britain in the forefront of the international peace-building efforts in the country.
It comes days after UK soldiers took over security in the volatile province of Helmand, southern Afghanistan.
Gen Richards, who is based in Kabul, takes over from Lt Gen Mauro del Vecchio, who has spent nine months in charge.
The Nato-led International Security Assistance Force has about 9,000 personnel, but plans to expand to about 21,000 troops by November.
Gen Richards BBC Radio 4's Today programme: "There are things we will do that are quite benign, such as ensuring the security of government officials as they move around the country, through to, if necessary, taking on Taleban opponents wherever they may be.
"We will be focusing more on spreading the writ of the government and if that means that we come up against, as we almost certainly will, opponents - some are criminals, some are characterised as Taleban but aren't actually, and others are actually Taleban - we will take them on."
He said this had not caused undue concern because "at the lower tactical level" the forces will "respond robustly" to ensure that soldiers' lives are properly protected.
Nato spokesman James Appathurai said the situation in Afghanistan would become more dangerous.
"The mission is going to change, but that is exactly why Nato has changed its posture. We are sending our force with much more equipment, with more robust rules of engagement and in big numbers."
Gen Del Vecchio said Nato would deploy more than 7,000 soldiers to southern Afghanistan in the next few months to counter a rise in attacks by Taleban militants.
British forces have been threatened by Taleban fighters, who have told the BBC they plan to target and kill British troops in Helmand.
Some British troops have already arrived in Helmand, with more than 3,300 due to be deployed there.
'Confident and ready'
The soldiers have replaced US forces as part of an expansion of peacekeeping operations by Nato.
As 120 soldiers from 3rd Battalion, the Parachute Regiment prepared to leave the UK on Wednesday, their commanding officer Lt Col Stuart Tootal said they were "confident and ready" for the tasks ahead.
"We will be undertaking a range of roles from force protection of the reconstruction teams, which will be rebuilding infrastructure such as roads, schools and hospitals, to providing security to the Afghan National Army as they seek to tackle the country's drug problem."
The Army's Helmand deployment, led by 16 Air Assault Brigade, will be completed by June.