But improved analysis of drone video could be used to pick out human targets for strikes, while also better identifying civilians to reduce the accidental killing of innocent people.

Without referring directly to the letter to Mr. Pichai, Google said in a statement on Tuesday that “any military use of machine learning naturally raises valid concerns.” It added, “We’re actively engaged across the company in a comprehensive discussion of this important topic.” The company called such exchanges “hugely important and beneficial,” though several Google employees familiar with the letter would speak of it only on the condition of anonymity, saying they were concerned about retaliation.

The statement said the company’s part of Project Maven was “specifically scoped to be for non-offensive purposes,” though officials declined to make available the relevant contract language. The Defense Department said that because Google is a subcontractor on Project Maven to the prime contractor, ECS Federal, it could not provide either the amount or the language of Google’s contract. ECS Federal did not respond to inquiries.

Google said the Pentagon was using “open-source object recognition software available to any Google Cloud customer” and based on unclassified data. “The technology is used to flag images for human review and is intended to save lives and save people from having to do highly tedious work,” the company said.

Some of Google’s top executives have significant Pentagon connections. Eric Schmidt, former executive chairman of Google and still a member of the executive board of Alphabet, Google’s parent company, serves on a Pentagon advisory body, the Defense Innovation Board, as does a Google vice president, Milo Medin.

In an interview in November, Mr. Schmidt acknowledged “a general concern in the tech community of somehow the military-industrial complex using their stuff to kill people incorrectly, if you will.” He said he served on the board in part “to at least allow for communications to occur” and suggested that the military would “use this technology to help keep the country safe.”

An uneasiness about military contracts among a small fraction of Google’s more than 70,000 employees may not pose a major obstacle to the company’s growth. But in the rarefied area of artificial intelligence research, Google is engaged in intense competition with other tech companies for the most talented people, so recruiters could be hampered if some candidates are put off by Google’s defense connections.