There is concern the Taliban may have infiltrated the Afghan police force after the killing of five British soldiers at a checkpoint.
Authorities say a local policeman opened fire, killing the five soldiers and seriously wounding five others.
Lt Col David Wakefield said: “Our initial understanding of what happened yesterday appears to be an individual, a rogue who commited this. We are putting every effort, as you can imagine, into tracking this man down and those responsible for it.”
The killings happened at a military compound in Helmand province. Other British soldiers returned fire, but the rogue policeman escaped.
British Prime Minister Gordon Brown said: “The death of five brave soldiers in a single incident is a terrible and tragic loss. And I want to pay tribute, as the whole house will, to their professionalism, courage and service.”
Gordon Brown had already faced criticism over troop numbers, tactics and equipment levels.
The shooting came a day after Afghan officials cancelled a run-off vote in disputed presidential elections and gave President Hamid Karzai a second term in office.