British troops have been deployed near the Pakistan border to stop al-Qaeda and Taliban fighters who may try to re-enter Afghanistan.
Some 300 Royal Marine commandos have been deployed over the past three days in the area, but a senior British official said the number could go up to 700.
British forces will conduct patrols by helicopter, foot and vehicle in Operation Buzzard.
"The key point is being unpredictable, operating overtly and covertly to introduce doubts in the minds of the al-Qaeda and Taliban," an army spokesman said.
Their mission is to disrupt al-Qaeda and Taliban infiltration routes and safe havens, to deny their freedom of manoeuvre, deny them sanctuary and gather intelligence.
The operation is being launched at a time that coalition officials warn that al-Qaida and Taliban may launch insurgency attacks, including suicide bombings.
It could also try and disrupt next month's meeting by the loya jirga, or grand council, to choose a transitional government.
PA