As many as 12,000 Taliban fighters have been killed, captured or wounded in Afghanistan in the past year, according to a UN report which warns that despite Afghan military successes, the insurgency will remain resilient as long as it enjoys a wide range of illicit income streams.

According to Afghan government sources and "Taliban internal statistics" quoted in a special report to the UN security council, between 10,000 and 12,000 rebels are thought to have been killed, captured or wounded. Deaths and injuries among the Afghan police and army have also soared in the last year. A UN official said the number of Taliban losses was a threefold increase on the figure for the same period last year.

The US-led Nato alliance has long refused to publish information about the number of Taliban fighters it believes have died on the battlefield.

The report, by the committee in charge of the UN's list of senior members of the Taliban subject to international sanctions, says Afghanistan's army and police have performed well in the past 12 months, even succeeding in taking back some Taliban-controlled territory.

A Taliban effort to overrun towns in 2013 had not "led to significant gains for the Taliban, who have neither managed to seize population centres nor gain popular support", the report says.

But with violence in the country soaring to levels "not seen since 2010", the report gives little hope of a respite in fighting as the country prepares for the end of the Nato combat mission next year.

A gathering of tribal elders in Kabul will this week debate whether any foreign forces will be allowed to remain after 2014 to support Afghan troops.

The Taliban remains a powerful and well-funded force, the report says, with the movement raising $155m in 2012 from illegal opium production.

Although the amount of protection money that insurgents receive from security companies employed to guard Nato supply convoys has fallen as foreign forces close bases, the report says 2014 is expected to be a bumper year as the alliance ships huge amounts of equipment out of the country.

It also warns that the Taliban is skimming profit off illegally mined gemstones, including rubies and emeralds. Afghanistan has an estimated $1tn worth of mineral reserves, which it is hoped will eventually help to pay for the country's 352,000-strong security force.

The report says Kabul needs to do much more to prevent high-grade industrial explosives reaching the hands of Taliban bomb-makers, whose weapons are becoming "increasingly sophisticated and technically advanced" and now account for 80% of army and police casualties.

The training of suicide bombers outside Afghanistan is described as "a particularly worrying trend", with the report citing two cases of suicide attackers sexually abused "in order to be conditioned for their mission". It also reports an increase in the reliance on pistols with silencers, which have been "used widely in intimidation campaigns" during 2013.

On Sunday, the bodies of six government building contractors were found beheaded in the southern province of Kandahar.

The report says that although there is still a chance of the Taliban entering peace talks, it warns that "those interested in dialogue still appear subordinate to those committed to further fighting".

It also says the insurgency is becoming increasingly fragmented, with the rise of a new generation of commanders operating their own "fronts" that are largely independent of the movement's leadership.