One of the Taliban's most senior commanders has admitted the insurgents cannot win the war in Afghanistan and that capturing Kabul is "a very distant prospect", obliging them to seek a settlement with other political forces in the country.

In a startlingly frank interview in Thursday's New Statesman, the commander – described as a Taliban veteran, a confidant of the leadership, and a former Guantánamo inmate – also uses the strongest language yet from a senior figure to distance the Afghan rebels from al-Qaida.

"At least 70% of the Taliban are angry at al-Qaida. Our people consider al-Qaida to be a plague that was sent down to us by the heavens," the commander says. "To tell the truth, I was relieved at the death of Osama [bin Laden]. Through his policies, he destroyed Afghanistan. If he really believed in jihad he should have gone to Saudi Arabia and done jihad there, rather than wrecking our country."

The New Statesman does not identify the Taliban commander, referring to him only as Mawlvi but the interview was conducted by Michael Semple, a former UN envoy to Kabul during the Taliban era who has maintained contacts with members of its leadership, and served on occasion as a diplomatic back-channel to the insurgents.

Semple, who is now at the Carr Centre for Human Rights Policy at Harvard, said the commander's identity had to be protected because the Taliban was highly sensitive about unauthorised pronouncements on the movement's behalf, but he added there was no doubt about Mawlvi's role within the movement.

"I maintain dialogue over time rather than have one-off contacts so I know who Mawlvi is and I know everyone he is talking to," he said.

Semple said that speaking unofficially allowed Mawlvi to stray from the rigidly controlled Taliban "party line" and voice the unvarnished views of a pragmatic wing of the leadership, which Semple describes as "making a serious bid to shape the strategy of the movement".

Mawlvi's scepticism over his own side's military prospects is in particularly striking contrast to the consistently triumphalist output of official Taliban statements. "It is in the nature of war that both sides dream of victory. But the balance of power in the Afghan conflict is obvious. It would take some kind of divine intervention for the Taliban to win this war," he says.

"The Taliban capturing Kabul is a very distant prospect. Any Taliban leader expecting to be able to capture Kabul is making a grave mistake. Nevertheless, the leadership also knows that it cannot afford to acknowledge this weakness. To do so would undermine the morale of Taliban personnel. The leadership knows the truth – that they cannot prevail over the power they confront," Mawlvi says.

As a result, he says that the Taliban has had to shelve its dream of re-establishing the Islamic emirate it set up when it was in power from 1996 to 2001. "Any side involved in a conflict like this has decided to fight for power. If they fall short of achieving national power, they have to settle for functioning as an organised party within the country," he admits.

He is scathing about President Hamid Karzai, who the Taliban has consistently derided as a US puppet. "There is little point in talking to Kabul. Real authority rests with the Americans," he says. "The only other serious political force in Afghanistan is that of the Northern Alliance" – a Tajik-led coalition that led the resistance to Taliban rule and is now a powerful player in Kabul.

David Miliband, who was an early champion of talking to the Taliban when he was foreign secretary, said the interview represented an opportunity that should be seized. "This landmark interview shows both the need for and difficulties in serious discussion with the Taliban about the future of Afghanistan," Miliband, who published the interview as the guest editor of the Statesman, argued.

"The candour and clarity of the remarks about al-Qaida, Nato and the Afghan government show that we are dealing with a sophisticated and long-term presence in the country that cannot be wished away," he said. "With 10,000 British troops in the country it is vital that those talks are taken forward now. Afghanistan cannot become the forgotten war."

Earlier this year, the Taliban sent representatives to Qatar to act as a political office for negotiations with the US. However, the talks soon stalled largely because of resistance to such contacts from Karzai, who felt he had been excluded, and reluctance in Washington to authorise the transfer of five Taliban prisoners in Guantánamo, something the Taliban had been led to believe had been agreed in preliminary talks as a confidence-building measure.

The Taliban officially suspended the contacts in March but kept its envoys in Qatar. It also sent a delegation last weekend to a reconciliation conference in Kyoto. In the article, Mawlvi signals that the Taliban's pragmatic wing at least remains committed to the talks.

"The world has long been keen to portray the Taliban as wild and uncivilised, ignorant of international norms and uninterested in government. Nato has long claimed that it wants peace but the Taliban are an obstacle who refuse to break links with al-Qaida. The Taliban wanted to turn the tables on Nato and show who are the real obstacles to peace," he says.

Mawlvi maintains the Taliban interest in negotiations goes beyond the immediate desire to get its men out of Guantánamo. If that had been the case, they would not have bothered going to Qatar but would simply have established a commission for prisoner exchange, he said.

Semple says it is hard to judge the influence of pragmatists such as Mawlvi in comparison to more radical jihadists grouped around the overall leader, Mullah Omar. Mawlvi's outspoken contempt for al-Qaida conflicts with evidence found in Osama Bin Laden's hideout in Abbottabad pointing to close working relationship between Omar and al-Qaida's leadership in orchestrating attacks on Pakistan and Afghanistan.

Semple argues that greater western commitment to talks would help the movement disentangle itself from al-Qaida. Mawlvi dismisses what he says are the few hundred al-Qaida fighters still in the region as irrelevant, saying the Taliban had not made a formal break only because it feared "it might alienate some Islamist constituencies".

It is also unclear whether the largely Pakistan-based Taliban leadership still has control over junior field commanders in Afghanistan, who have become progressively younger and more radical as a result of an intensive campaign of assassination spearheaded by US and British special forces over recent years.

"In truth, no one knows whether the Taliban leadership has the authority to make a peace deal," Mawlvi says. "But the same question could well be asked about Karzai, except that, with regard to Kabul, we know that authority is in the hands of someone else."