Nato and the United Nations are cautiously considering a Taliban proposal to set up a joint commission to investigate allegations of civilians being killed and wounded in the conflict in Afghanistan, diplomats in Kabul have told the Guardian.

The Taliban overture, which came in a statement posted on its website, will revive a divisive debate about whether to conduct any formal talks with insurgents who are responsible for the majority of civilian casualties in Afghanistan, and whose assassination campaign now kills one person a day on average.

The Taliban statement called for the establishment of a body including members from the Organisation of the Islamic Conference, UN human rights investigators, Nato and the Taliban.

"The stated committee should [be] given a free hand to survey the affected areas as well as people in order to collect the precise information and the facts and figures and disseminate its findings worldwide," the Taliban said.

One human rights organisation has already thrown its support behind the joint commission plan, which echoes a similar idea floated four years ago.

The UN and Nato are treading carefully, but western diplomats say the proposal is being carefully considered. One said that some senior officers at the Nato-led International Security Assistance Force (Isaf) were keen on the idea but that no steps could be taken until it was considered "at the highest political level".

The attraction to Nato is that contacts with the Taliban might help improve a dire security situation that threatens to deteriorate.

The Taliban statement also complained that data on civilian casualties were being used as "propaganda by the western media".

A UN report last week showed that a 31% rise in civilian casualties was largely caused by insurgents, who have increasingly turned to homemade bombs and assassinations. The report also destroyed one of the Taliban's main propaganda claims: that foreign forces are responsible for most civilian deaths and injuries.

In fact, the report showed "pro-government" forces were responsible for just 12%, while insurgents were responsible for 76%.

A 30% drop in the number of casualties caused by foreign forces was put down to severe restrictions on the use of heavy weapons and air strikes.

Today meanwhile it emerged that Taliban forces had resorted to another dark tactic barely used in recent years. A couple accused of adultery were stoned to death in the northern Kunduz province. Amnesty International said the killing underscored a basic fundamental: "That the Afghan government should not sacrifice human rights, particularly the rights of women and minorities, in the name of reconciliation with the Taliban and other insurgent groups."

Today Afghanistan's president, Hamid Karzai, indicated that the private security industry that guards everything from Nato supplies to foreign embassies would be scrapped within four months and all security handed over to the Afghan police. The announcement appeared likely to widen the rift between Karzai and his western backers.

The delicate balancing act for Nato is that the possible benefits of opening dialogue with insurgents must be weighted against the danger of simply giving them political legitimacy at a time when David Petraeus, the US commander of Nato forces, has ordered his communications department to cast the Taliban in the most negative light possible.

Today Nato military spokesmen put out statements attacking the Taliban's record on civilian casualties, but refused to comment on whether the plan was being ruled out. Isaf said: "The Taliban clearly realise their vulnerability on the issue of civilian casualties, and are trying to devise a way to evade responsibility for them. But the facts are clear – the Taliban have caused the overwhelming majority of civilian casualties."

A spokesman for the UN, which has for years had unofficial contacts with insurgents and sees itself as an independent arbitrator in the conflict, simply said: "We are aware of the statement and we are considering it."

There are some concerns, shared by the UN, about whether the proposal, which was posted under the name of a Taliban spokesman, Zabiullah Mujahid, actually comes from the movement's leadership. "Although it was on their website and from their spokesman, this is actually low-level and there has been no other reach out," a diplomat said. And there are fears that the Taliban have not shown contrition for the huge number of people killed and injured by improvised explosive devices and their assassination campaign, which, according to last week's UN report, now runs at an average of one person killed a day.

Afghanistan Rights Monitor, an organisation that investigates human rights abuses, said in a statement that it welcomed the Taliban's announcement, but issued a list of 12 provisos.

They included a demand the Taliban provide safety guarantees to human rights investigators in areas under their control and an immediate stop to suicide attacks in areas where civilians are concentrated.

Eric Gaston, a human rights lawyer and Afghanistan specialist at the Open Society Institute, said: "This offer would be more credible if the [Taliban] demonstrated a commitment to reducing civilian casualties itself by not attacking military targets indiscriminately, nor targeting and assassinating civilian officials and persons."