Peace will only come to Afghanistan if Pakistan can son out the militants on its side of the border, where US strikes are not helping, the head of Britain's armed forces told The Sunday Times newspaper.

Air Chief Marshal Sir Jock Stirrup said only politics, long term, could bring peace on both sides of the frontier.

The chief of the defence staff said that weaknesses in Afghan President Hamid Karzai's government were causing difficulties for the 8,300 British troops battling Taliban insurgents in the troubled south of the country.

"The weakness of governance in Afghanistan worries me considerably," Stirrup told the weekly broadsheet.

"But governance is not just about what goes on in Kabul. We have to look at the wider picture.

"The Taliban movement -- and Taliban is now a catch-all phrase for ideologues, criminals, people with tribal grudges, people who are quite simply guns for hire to keep bread on the table — is on both sides of the border.

"It makes no distinction between one side or the other. Some people move across. Some are based almost exclusively in Pakistan. Some are based exclusively in Afghanistan.

"It's impossible to distinguish between those two and actually, in my view, not necessary. The border is not relevant."

Stirrup sympathised with the difficulties faced by the Pakistani military, admitting that its success so far had been "limited".