New Delhi, Mar 5 (IANS): Intense diplomatic pressure exerted by India with the help of other countries led to the release of Wing Commander Abhinandan Varthaman by Pakistan, informed sources said here on Tuesday and noted that India had made it clear that the issue was non-negotiable.

They said that conversations with different countries were held at various levels including by Prime Minister Narendra Modi, and the release was an outcome of pressure on Pakistan premier Imran Khan and not a result of any generosity on his part.

Varthaman, who was captured by Pakistan after his MiG-21 crashed in Pakistan-occupied Kashmir following an aerial engagement with Pakistan Air Force (PAF) jets, returned to India on Friday night. Varthaman had shot down a F-16 fighter of Pakistan Air Force during the aerial engagement.

The sources said that India had conveyed to the five permanent members of the UN Security Council as also some other friendly countries that Varthaman's safe and immediate return was non-negotiable.

Khan had announced Varthaman's return as a "peace gesture" to India.

They said that perhaps Imran Khan was not aware of the full facts concerning the aerial engagements and Islamabad has also given no evidence in support of its claim that it had shot down a Sukhoi 30 fighter jet, as it was a lie. They said that all Sukhoi 30s in Indian Air Force's inventory were intact.

The sources also referred to reports in the Pakistani media that the piece of an AMRAAM missile recovered from Rajouri in Jammu and Kashmir was of Taiwanese origin, and said that Taiwan had refuted such reports.

The missile piece was shown during a joint briefing by the three services last week to refute Pakistan's claims that it had not used F-16 fighter jets in the aerial engagement.

AMRAAM missile can only be carried by F-16 procured by Pakistan from the US.

The sources said that it was for the US to investigate into any violation by Pakistan of the sale terms concerning use of F-16 aircraft.