The letter came months after the Indian Air Force in February shot down an F-16 jet of the Pakistan Air Force during a dogfight over Jammu and Kashmir in the aftermath of Indian Air Force strike in Balakot on terror camps inside Pakistan following Pulwama bombing.
It was written by Andrea Thompson, the then-Undersecretary of State for Arms Control and International Security Affairs, and said that it served as a direct response to US concerns about the F-16 use over Kashmir, the US News & World Report website said in its report published on Wednesday.
The letter itself, however, did not specifically refer to the incident.
Addressed to the head of the Pakistani Air Force, Air Chief Marshal Mujahid Anwar Khan, the letter began by relaying the State Department's confirmation that Islamabad had moved the F-16s and accompanying American-made missiles to unapproved forward operating bases in defiance of its agreement with Washington.
Thompson, who has since left government, warned Pakistan that their behaviour risked allowing these weapons to fall into the hands of malign actors and "could undermine our shared security platforms and infrastructures".
The letter represents the first admission since February from the US of its concerns about how Pakistan used its fleet of F-16s in stark violation of the original terms of the sale, the report said.
A State Department spokeswoman had said in March that it acknowledged the Indian reports of Pakistan's misusing the fighters in the February skirmish, adding "we're following that issue very closely".
Regarding Wednesday's report, neither the State Department has responded nor the Pakistan Embassy in Washington D.C.
The dogfight took place following the February 14 Pulwama suicide bombing which claimed the lives of 40 CRPF personnel and was claimed by the Pakistan-based Jaish-e-Mohammad (JeM) terror group.
In response, India attacked JeM training camps in Balakot, Pakistan.
( With inputs from IANS )