By Hamid Shalizi

KABUL (Reuters) - The former head of Afghanistan's main intelligence agency released documents on Thursday which he said showed that Pakistani intelligence services helped leaders of the Taliban and the feared Haqqani network in 2014 and 2015.

Rahmatullah Nabil stepped down from the National Directorate of Security (NDS) in December last year after opposing Afghan President Ashraf Ghani's efforts to improve relations with Pakistan and include Islamabad in peace talks with the Taliban.

Nabil told a group of journalists in Kabul that he had released the documents to provide concrete evidence of Pakistan's collusion with the Taliban and the associated Haqqani group, which has been blamed for a series of kidnappings and high profile suicide bombings in the capital.

Pakistan's foreign office and the army's media wing did not respond to several written and telephoned requests for comment in which Reuters outlined the accusations leveled against Pakistan.

Neither the NDS nor the Afghan government was available for immediate comment on the letters.

The Afghan Taliban insurgency has in the past said it is fighting against the government and Western allies with the support of the Afghan people, and has no need of outside help.

Nabil did not say how the letters had been obtained. Reuters could not independently verify their authenticity.

Since retiring from the intelligence service, Nabil has been strongly critical of Pakistan, which is routinely accused by Afghanistan of sponsoring the Afghan Taliban, a charge it has consistently denied.

"For the past 14 years, no one has disclosed documents of this kind. Here, I'm proving it," he told reporters, to whom he released the letters. "They kill us every day and commit all kinds of atrocities, we have to show them."

One letter, addressed from a section of Pakistan's military intelligence service in the northwestern city of Peshawar, is headed "Arrangements of Secure Houses and Protection to Afghan Taliban and Their Leadership".

In the letter, dated August 2014, an official arranges for safe houses and vehicles to be provided for Afghan Taliban commanders forced out of a remote area of northern Pakistan while an army operation is conducted.

Another letter, dated March 2015, requests an update on Haqqani network personnel in Nowshera, Mardan and Swabi, in the border province of Khyber Pakhtunkhwa.

A third letter addressed from the Directorate General Military Intelligence, Ministry of Defence, dated July 2014, is headed "Kabul Airport Attacks and Release of Payments".

The letter says four members of the Haqqani network are to be paid 2.5 million Pakistani rupees ($24,000) each for the "successful and comprehensive execution of assault on KB AP".

VICTIM OF TERRORISM

Authorities in Islamabad have long rejected accusations that Pakistan has provided support and sanctuary for the Afghan Taliban and its leaders, saying it has for years itself been a victim of terrorism, much of it from groups based in Afghanistan.

The release of the documents comes amid worsening tensions between the neighboring countries in recent weeks, with days of clashes last month during which troops on the border exchanged artillery fire.

It also came days after Pakistan faced heavy criticism in a Congressional Foreign Affairs Committee hearing in Washington.

Afghan President Ghani sharply criticized Pakistan during a visit to a NATO summit in Warsaw last week, and has accused it of conducting an "undeclared war" against Afghanistan.

In response, Islamabad said that Afghanistan was playing a "blame game" instead of cooperating effectively to stop terrorism.

Pakistan's critics say that it supports Islamist militants in Afghanistan to maintain influence and counter attempts by arch-rival India to gain a foothold, effectively surrounding it.

(Additional reporting by Drazen Jorgic and Mehreen Zahra-Malik in Islamabad; Writing by James Mackenzie; Editing by Mike Collett-White)