Pakistan's powerful intelligence services were heavily involved in preparations for the Mumbai terrorist attacks of November 2008, according to classified Indian government documents obtained by the Guardian.

A 109-page report into the interrogation of key suspect David Headley, a Pakistani-American militant arrested last year and detained in the US, makes detailed claims of ISI support for the bombings.

Under questioning, Headley described dozens of meetings between officers of the main Pakistani military intelligence service, the ISI, and senior militants from the Lashkar-e-Taiba (LeT) group responsible for the Mumbai attacks.

He claims a key motivation for the ISI in aiding the attacks was to bolster militant organisations with strong links to the Pakistani state and security establishment who were being marginalised by more extreme radical groups.

Headley, who undertook surveillance of the targets in Mumbai for the operation, claims that at least two of his missions were partly paid for by the ISI and that he regularly reported to the spy agency. However, the documents suggest that supervision of the militants by the ISI was often chaotic and that the most senior officers of the agency may have been unaware at least of the scale and ambition of the operation before it was launched.

More than 160 people were killed by militants from LeT who arrived by sea to attack luxury hotels, a Jewish centre, a café, a hospital and the main railway station in Mumbai, the Indian commercial capital. Casualties included citizens from 25 countries, including four Americans killed and seven Britons injured. The attacks dominated media for days and badly damaged already poor Indian-Pakistan relations.

European and American security services now fear that LeT, which has thousands of militants, runs dozens of training camps and has extensive logistic networks overseas, is moving from what has been a largely regional agenda – focused on the disputed Himalayan former princely state of Kashmir – to a global agenda involving strikes against the west or western interests. The documents suggest the fierce internal argument within the organisation over its strategic direction is being won by hardliners.

Headley, interviewed over 34 hours by Indian investigators in America in June, described how "a debate had begun among the terrorist outfits" and "a clash of ideology" leading to "splits".

"The aggression and commitment to jihad shown by several splinter groups in Afghanistan influenced many committed fighters to leave [LeT]," Headley said. "I understand this compelled the LeT to consider a spectacular terrorist strike in India."

Headley, who changed his name from Daood Gilani, told the investigators that the ISI hoped the Mumbai attack would slow or stop growing "integration" between groups active in Kashmir, with whom the agency had maintained a long relationship, and "Taliban-based outfits" in Pakistan and Afghanistan which were a threat to the Pakistani state.

"The ISI … had no ambiguity in understanding the necessity to strike India," Headley is reported to have said. The aim of the agency was "controlling further split in the Kashmir-based outfits, providing them a sense of achievement and shifting … the theatre of violence from the domestic soil of Pakistan to India."

Headley describes meeting once with a "Colonel Kamran" from the military intelligence service and having a series of meetings with a "Major Iqbal" and a "Major Sameer Ali". A fellow conspirator was handled by a Colonel Shah, he claims. Headley also alleges that he was given $25,000 by his ISI handler to finance one of eight surveillance missions in India.

However, Headley describes the ISI director general, Lt General Shuja Pasha, visiting a key senior militant from LeT in prison after the attacks in a bid "to understand" the operation, implying that, as many western security agencies suspect, the top ranks of the agency were unaware of at least the scale of the planned strike.

The Pakistani government has repeatedly denied any involvement of any security official in the Mumbai attacks. Last night, an ISI spokesman told the Guardian the accusations of the agency's involvement in the Mumbai attacks were "baseless".

LeT was banned in Pakistan in 2002. Jamat-ud Dawa, the social welfare wing of LeT, has been blacklisted in the wake of the Mumbai attacks although it continues to function.

The revelations could prove embarrassing to the US government as well as to the Pakistanis. Reports in American newspapers over the weekend claimed that Headley's wife had tried to alert American authorities to her husband's activities but had been ignored.