Britain's former ambassador to Afghanistan has attacked the conduct of the war by the US commander, General David Petraeus, describing the future CIA chief's tactics as counter-productive and "profoundly wrong".

Sir Sherard Cowper-Coles, who also served as the UK's special envoy for Afghanistan and Pakistan, added that Petraeus should be "ashamed of himself" for making claims of the number of insurgent commanders his forces had killed.

"He has increased the violence, trebled the number of special forces raids by British, American, Dutch and Australian special forces going out killing Taliban commanders, and there has been a lot more rather regrettable boasting from the military about the body count," said Cowper-Coles. He added that the use of statistics was reminiscent of the Vietnam war. "It is profoundly wrong and it's not conducive to a stable political settlement."

Petraeus is due to leave Afghanistan to become CIA director this summer. Since taking command of US and coalition troops in Afghanistan last June, he has increased the use of special forces raids and drone attacks against Taliban commanders.

Earlier this year, Petraeus told Congress that his forces were killing or capturing 360 insurgent leaders every three months. His officers argue that the tactic is demoralising the Taliban and will ultimately make the movement more likely to agree to a peace deal on the terms of Kabul and the west.

Cowper-Coles insists the tactic will make it harder for the west to find a political settlement and end the war. "There is no doubt that Petraeus has hammered the Taliban extremely hard," he said. "I am sure that some of them are more willing to parlay. But, equally, for every dead Pashtun warrior, there will be 10 pledged to revenge.

"Of course it produces tactical success in cleansing insurgents out of particular areas, but it's essentially moving water around a puddle, and I think any general who boasts of the number of Pashtun insurgents he's killed should be ashamed of himself."

He added: "Regrettably, General Petraeus has curiously ignored his own principles of counter-insurgency in the field manual, which speaks of politics being the predominant factor in dealing with an insurgency."

He compared the US commander unfavourably with his predecessor, General Stanley McChrystal, whose central approach was to protect Afghan civilians, even if meant greater caution in the pursuit of the Taliban.

Alongside the former foreign secretary, David Miliband, Cowper-Coles focused his efforts while UK special envoy on persuading the Obama administration to concentrate on a political settlement and start talking to the Taliban.

Some reports suggest that Washington has initiated such contacts. But British officials say that Marc Grossman, the US special envoy on Afghanistan and Pakistan leading the outreach effort, is having trouble finding any credible Taliban representatives to engage in even talks about talks.

Few serving British and European officials are as critical of Petraeus as Cowper-Coles. Most argue that the Taliban have to be put under some kind of focused military pressure to persuade them that a negotiated settlement was in their interest.

However, there is growing unease in Whitehall that, despite orders to the contrary from Obama and the US secretary of state, Hillary Clinton, the military effort was still taking priority.

"There are different parts of the Washington establishment who are pulling in different ways," one official said. "But as long as Petraeus is in Kabul, the military approach will take precedence."

Petraeus is expected to leave Afghanistan in September. In any case, there are few expectations of much progress towards contacts with the Taliban until at least the end of the summer fighting season. Most serving officials are also less confident than Cowper-Coles that senior ranks in the Taliban are interested in a political settlement.

"In 2011, there have been more feelers coming out from more senior people, but there is no solid evidence that anyone in the movement has been tasked with finding a route to peace," one official said.

There have been several backdoor attempts to draw the movement into a dialogue, but they have made little progress. "Why would they negotiate?" asked Michael Scheuer, the former head of the CIA's Bin Laden unit and an expert on the Taliban. "They are winning; they are no longer ostracised in the Islamic world for links to Osama bin Laden. Why would you throw that away?"

But Scheuer, the author of a new book on Bin Laden, said that Petraeus's "decapitation" approach was also unlikely to work."The Red Army tried that for 10 years, and they were far more ruthless and cruel about it than us, and it didn't work so well for them," Scheuer said.