In the past six months about 25 British soldiers have been killed in Helmand province, where most of Britain's 7,000 troops are deployed.
Another 800 soldiers are due to join the British contingent in Isaf by the end of the year.
Also in Afghanistan, negotiators made contact on Friday with the captors of four workers from the International Committee of the Red Cross (ICRC).
The men, two of whom are foreigners, were on a mission to free a German kidnapped by the Taliban when they were seized on Wednesday in Wardak province, about 50km from the capital, Kabul.
|The ICRC has been involved in missions to free|
those abducted in Afghanistan [AFP]
"The Red Cross office advised us not use any military action for the safety of the kidnapped people and the issue must be solved via mediation through tribal elders," Anayatullah Mangal, the governor of Sayed Abad district, said.
"We are in contact with the kidnappers via tribal elders and influentials," Mangal said.
The Red Cross workers did not return to Kabul on Wednesday after their mission in Wardak, where a 62-year-old German engineer and five Afghans were captured 10 weeks ago.
The ICRC has played a role in securing the release of some Taliban hostages, including 21 South Korean Christian aid workers captured in mid-July and released in August.
Besides the two Afghans, one of the men was from Myanmar and another from Macedonia, the ICRC said, adding that the workers were expected to be freed soon.
"We are in contact with all the involved parties," Graziella Leite Piccolo, an ICRC spokesperson, said in Kabul.
An "armed group" was involved in the abduction, she said, without elaborating.
The Taliban said on Friday that it was not involved in the disappearance of the four Red Cross workers.
Yousuf Ahmadi, who identified himself as the Taliban's main spokesman, said: "This is not our work. I cannot say anything at this stage [about] who might have done it."
The Afghan government said on Thursday it had arrested Ahmadi, but a man who sounded like Ahmadi later called the media to say he was still free.