A top American diplomat sent a written reprimand to the chiefs of the Pakistani air force in August accusing them of misusing U.S.-supplied F-16 fighter jets and jeopardizing their shared security, according to documents obtained by U.S. News.

The communication came months after India claimed one such F-16 shot down one of its fighter jets during a days-long skirmish in February over the contested region of Kashmir, which would amount to a fundamental violation by Pakistan of the terms governing the sale of its U.S. fighter jets and a dangerous form of military escalation among nuclear powers.

A source who viewed the August letter, written by Andrea Thompson, then-undersecretary of State for Arms Control and International Security Affairs, says it serves as a direct response to U.S. concerns about the F-16 use over Kashmir in February, though the letter itself does not specifically reference 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 Pakistan had moved the F-16s and accompanying American-made missiles to unapproved forward operating bases in defiance of its agreement with the U.S. Using diplomatic language, Thompson, who has since left government, warned the Pakistanis that their behavior risked allowing these weapons to fall into the hands of malign actors and "could undermine our shared security platforms and infrastructures."

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The letter represents the first admission since February from the U.S. of its concerns about how Pakistan used its fleet of F-16s in stark violation of the original terms of the sale. A State Department spokeswoman said in March that the department acknowledged the Indian reports of Pakistan's misusing the fighters in the February skirmish, adding "we're following that issue very closely."

The State Department declined to respond to questions on the record. An official speaking on the condition of anonymity said the department as a matter of policy does not comment publicly "on the contents of bilateral agreements involving U.S. defense technologies, nor the communications we have about them."

The Pakistani Embassy in Washington, D.C. did not respond to requests for comment.

Several diplomatic officials and analysts with experience in Pakistan say it is not surprising that Thompson did not expressly mention in the message U.S. concerns about using the F-16s to shoot down the Indian fighter jet. Acknowledging in a formal State Department transmission such a clear violation of the congressionally approved terms for selling the fighter jets to Pakistan would likely have triggered formal procedures to reprimand Islamabad at a time the Trump administration is attempting to repair contentious relations with the ally.

Thompson, a career military intelligence officer who first entered the administration as Vice President Mike Pence's national security adviser, admonished Pakistan in the letter for having "relocated, maintained and operated" the American made F-16s and the AMRAAM missiles they use from forward operating bases not approved under the original terms of the sale. The former Army colonel, who left the White House in September , also expressed concern at the access Pakistani officials allowed American weapons inspectors.

"While we understand from you that these aircraft movements were done in support of national defense objectives," Thompson wrote in the letter, "the U.S. government considers the relocation of aircraft to non-U.S. government authorized bases concerning and inconsistent with the F-16 Letter of Offer and Acceptance."

"Such actions could subject sensitive U.S.-technologies to diversion to or access by third parties, and could undermine our shared security platforms and infrastructures," Thompson wrote.

A flare-up in military tensions between Pakistan and India began in mid-February, after a Pakistani militant group claimed credit for a suicide bombing in Kashmir that killed 40 Indian security personnel. India has consistently claimed that Pakistan uses militants to destabilize the region, which Pakistan and India have each claimed since they were separated by partition in 1947.

The subsequent tensions escalated as both countries deployed fighter jets, and in one dogfight an Indian plane was shot down. Its pilot landed in Pakistani territory and was imprisoned until his release in March. On Feb. 28, the Indian government presented evidence it says showed Pakistani jets fired AMRAAM missiles at the Indian planes.

The Pakistani armed forces possess 76 American-supplied F-16s – by far the most potent fighter jet in its military arsenal. Pakistan first began receiving the plane in 1982 and maintains them under strict rules imposed by the State Department, the Department of Defense and Congress. Among the rules are that Islamabad may only house the fighters and the corresponding American missiles on two specific air force bases at Mushaf and Shahbaz and that it only uses them for counter-terror operations, not against foreign countries.

The agreement for their sale and subsequent operation, governed in part by the State Department's Defense Security Cooperation Agency, also stipulates that American contractors and mechanics must have access to the jets at any time of day or night both to help maintain them and to monitor how the Pakistani military employs them.

The agency in July – weeks before Thompson's letter – re-approved the terms for these monitors, known as Technical Security Teams, at a cost of $125 million.

"This proposed sale will support the foreign policy and national security of the United States by protecting U.S. technology through the continued presence of U.S. personnel that provide 24/7 end-use monitoring," the agency wrote in a statement announcing the renewed contract, which must receive congressional approval.

Those who track aerial combat in the region and the weapons used for it aren't surprised that Pakistan would risk being caught violating its agreement with the U.S. when it regards an issue as hotly contested as Kashmir.

"Given how volatile the situation was, it was important for both sides not to lose face in getting their plane shot down," says Karl Kaltenthaler, a professor at the University of Akron. "It makes sense that Pakistan would do that, but it was at the potential cost of getting called out by the U.S. for using the weapons platform that way. For the Pakistanis, this is how they operate."