A stream of US military intelligence reports accuse Pakistan's Inter-Services Intelligence (ISI) spy agency of arming, training and financing the Taliban insurgency since 2004, the war logs reveal, bringing fresh scrutiny on one of the war's most contentious issues.

At least 180 files contain allegations of dirty tricks by the powerful agency with accounts of undercover agents training suicide bombers, bundles of money slipping across the border and covert support for a range of sensational plots including the assassination of President Hamid Karzai, attacks on Nato warplanes and even poisoning western troops' beer supply.

They also link the ISI to some of the war's most notorious commanders. In April 2007 for instance, the ISI is alleged to have sent 1,000 motorbikes to the warlord Jalaluddin Haqqani for suicide attacks in Khost and Logar provinces.

But for all their eye-popping details, the intelligence files, which are mostly collated by junior officers relying on informants and Afghan officials, fail to provide a convincing smoking gun for ISI complicity. Most of the reports are vague, filled with incongruent detail, or crudely fabricated. The same characters – famous Taliban commanders, well-known ISI officials – and scenarios repeatedly pop up. And few of the events predicted in the reports subsequently occurred.

A retired senior American officer said ground-level reports were considered to be a mixture of "rumours, bullshit and second-hand information" and were weeded out as they passed up the chain of command. "As someone who had to sift through thousands of these reports, I can say that the chances of finding any real information are pretty slim," said the officer, who has years of experience in the region.

If anything, the jumble of allegations highlights the perils of collecting accurate intelligence in a complex arena where all sides have an interest in distorting the truth.

"The fog of war is particularly dense in Afghanistan," said Michael Semple, a former deputy head of the EU mission there. "A barrage of false information is being passed off as intelligence and anyone who wants to operate there needs to be able to sift through it. The opportunities to be misled are innumerable."

The shaky intelligence does not mean the US does not believe the ISI is supporting the Taliban. The spy agency nurtured the Taliban in the 1990s and, although it purported to sever its ties after 9/11, is believed to maintain the relationship.

The British and US governments have repeatedly urged Pakistan to root out the Taliban from their sanctuary inside the border, with little effect. In July 2008 the deputy head of the CIA, Stephen Kappes, flew to Islamabad to reportedly confront the ISI with evidence that the agency orchestrated a suicide attack on the Indian embassy in Kabul that month which killed 54 people including the Indian defence attache. The CIA claimed to have intercepted phone conversations between ISI officers and the militants who carried out the attack.

Pakistani strategists see the Taliban as a useful proxy to marginalise the influence of arch-rival India. Indeed plots to attack Indian facilities in Afghanistan provide some of the most plausible allegations in the files. One report from November 2007 said the ISI was plotting an attack on the Indian consulate in Jalalabad; another, titled "ISI order murder and kidnappings", has the agency offering between $15,000 and $30,000 for the assassination of Indian road workers.

But many of the 180 reports appear to betray as much about the motivation of the sources than those of the alleged foreign puppet-masters. Some US officers were aware of this. One report from 2006 notes that an informant "divulges information for monetary remuneration and likely fabricated or exaggerated the above report for just that reason".

Some of the most striking claims come from the National Directorate of Security, Afghanistan's foremost spy agency and a bitter rival to the ISI.

In July and August 2008 the NDS passed information to the US that three Pakistan-trained militants plotting to kill Karzai had been groomed by a named ISI officer and had trained at the Zarb Momen camp outside Karachi. The attackers were Palestinian and Arab, the report said, and intended to strike during a visit by Karzai to a Kabul mosque or the luxury Serena hotel.

But the report's strong assertions fade under retrospective scrutiny. The predicted assault on Karzai never took place (the last reported attempt was in April 2008, four months earlier), and there is no known militant camp called Zarb Momen in Karachi, a city with hundreds of hardline madrasas. The al-Rashid Trust, a charity with militant links, publishes a magazine by the same name, said Amir Rana, an Islamabad-based militancy expert.

The miltiary's grading system offers one way of sifting the ISI file. Some 27 of the 180 reports are graded as C3 and above, meaning they come from a "fairly reliable source" and are "possibly true".

But many such reports appear highly implausible. In February 2007 the ISI and insurgents planned "to buy alcoholic drinks from markets in Miranshah [in Pakistan's tribal belt] and Peshawar [in order to] mix them with poison and use them for poisoning ANSF and ISAF troops" according to a C3 report. The Karzai plot is assessed to be "probably true".

Apparently more credible reports of ISI skulduggery are marked SEWOC, or Signals Intelligence Electronic Warfare Operations Centre, signifying they come from intercepted communications. One SEWOC report, in December 2007, accused the ISI of deploying children as suicide bombers. But the military source said that such intelligence was also prone to distortion, and that its value depended on whose conversation was being eavesdropped. "If we ever found out anything that the ISI or Pakistani military were somehow complicit in the insurgency, it never came from these sources. Never," he said.

One name that frequently surfaces is that of General Hamid Gul, director general of the ISI between 1987 and 1989, who is referenced in eight reports. One has him smuggling magnetic mines into Afghanistan to attack Nato troops; in another he is plotting to kidnap United Nations staff to bargain for imprisoned Pakistani militants. A report from January 2009 has Gul meeting Arab militants in Pakistan's tribal belt to send suicide vehicles into Afghanistan. "It was not known whether Hamid Gul was acting with the knowledge or consent of the ISI," the report states.

But while Gul, 73, is a well-known fundamentalist ideologue in Pakistan, experts say he is unlikely to play a frontline role in the fighting. Afghan informers may have used his name – he is notorious in Afghanistan – to spice up their stories, said Semple.

"There's a pattern of using a dramatis personae of famous ISI officers and Afghan commanders, and recurring reports of dramatic developments such as the delivery of surface-to-air missiles, to give these reports credibility," he said. "But most of them are simply fabricated."

Afghanistan has a long history of intelligence intrigues that stretches back to the early 19th century. Afghans have learned to use intelligence as a tool to influence the foreign powers occupying their land. In the past quarter century it has become a lucrative source of income in a country with few employment opportunities.

Since 2001 intelligence has become a tool to influence US policymakers, who enjoy the greatest military clout in the region but are poorly informed about its intricacies. The retired US officer said some NDS officials "wanted to create the impression that Pakistani complicity was a threat to the US". And more broadly speaking, "there's an Afghan prejudice that wants to see an ISI agent under every rock".

US generals are aware of the problem. In January Major General Michael Flynn said foreign newspaper articles about Afghanistan were more useful than the information collected by his own soldiers in the field. The huge intelligence apparatus in Afghanistan was "only marginally relevant" to Nato's overall war plan, he said. "We're no more than fingernail-deep in our understanding of the environment."