The Pentagon has put 50 of Afghanistan’s powerful opium barons on a “kill or capture” list, signalling a radical shift in tactics against the Taleban.

The announcement came as the commander of US forces in Afghanistan, General Stanley McChrystal, admitted that the insurgency, nurtured by tens of millions of dollars from the country’s vast poppy fields, now held the upper hand.

The existence of the “joint integrated prioritised target list” — a rogues’ gallery of drug lords who are earmarked for arrest or assassination — is revealed in an unpublished Senate report obtained yesterday by The Times.

It was confirmed by Rear-Admiral Gregory Smith, the top US military spokesman in Afghanistan. “The list of targets are those that are contributing to the insurgency — the key leadership — and part of that obviously is the link between the narco-industry and the militants,” he said.

The report by the Senate Foreign Relations Committee does not give names but it spells out the new military objectives clearly: “We have a list of 367 ‘kill or capture’ targets, including 50 targets who link drugs and insurgency,” a US general told the panel.

“The military places no restrictions on the use of force with these selected targets, which means they can be killed or captured on the battlefield,” the report states.

The generals insisted, however, that no targeted assassinations were authorised “away from the battlefield”. They did not provide their definition of “battlefield”.

Most of the drug lords linked to the Taleban are known to live in Quetta, Pakistan, and targeting them would probably have to involve operations similar to the missile strike last week that killed Baitullah Mehsud, the leader of the Pakistani Taleban.

Such cross-border strikes have become increasingly frequent in the war on the insurgency, often provoking the ire of the Pakistani Government. The new strategy of aiming Hellfire missiles at drug dealers, and even corrupt Afghan officials, as one officer suggested, will be even more controversial.

The Senate report notes that several Nato members opposed the tactic earlier this year, questioning “whether killing traffickers and destroying drug labs complied with international law”.

Two US generals told the panel that the rules of engagement governing the conduct of US forces in Afghanistan and the international law of war “had been interpreted to allow them to put drug traffickers with proven links to the insurgency on a kill list”.

The aggressive new approach to Afghanistan’s drug trade and its links to the militants reflects America’s frustration with the authorities in Kabul, the capital. The US previously sought the arrest of suspected drug lords but has found that they tended to get lost in the corrupt jungle that is the Afghan judiciary.

No extradition treaty exists between the two countries, so sometimes loopholes have to be found. The Senate report details one such “creative response” involving British and US agents.

“In October 2008, agents from the DEA [Drug Enforcement Administration] and Britain’s Soca [Serious Organised Crime Agency] tricked Haji Juma Khan, a major kingpin linked to the Taleban who ran his empire out of Quetta, into flying into Indonesia,” it says.

He was promptly arrested and flown to New York to face charges of conspiracy to distribute narcotics and supporting a terrorist organisation.

Estimates of the Taleban’s annual takings from the Afghan opium trade range between $75 million (£45 million) and $125 million.

According to the report, the largest source of the Taleban's drug income is the tax paid by traffickers to the Quetta shura, the insurgents' governing council based in the Pakistani city.

The urgency of curbing the Taleban’s power was highlighted by General McChrystal, who gave a bleak assessment of the progress of a war that has been dragging on for eight years. “It’s a very agressive enemy right now,” he said. “We’ve got to stop their momentum, stop their initiative. It’s hard work.”

As if to illustrate his point, a small group of Taleban fighters infiltrated a provincial capital 40 miles south of Kabul yesterday. The fighters - some disguised as women, with suicide vests hidden under their burqas - fired rocket-propelled grenades at government buildings in the city of Pul-i-Alam. Three policemen and two civilians were killed, along with six militants, said a local government spokesman.

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