Dragonfly is the name of the project that aims to build a search engine for the Chinese market, which would censor some websites and search queries like religion or human rights.
At a town hall gathering of employees on Thursday, Google CEO Sundar Pichai said the firm was committed to transparency, and that while it was "exploring many options", it was "not close to launching a search product in China", the Financial Times reported, citing a person present at the meeting.
The letter is similar to one thousands of employees had signed in protest of Project Maven, a United States military contract that Google decided in June not to renew.
More than 1,000 Google employees protested the tech giant's secretive plans to build a search engine for China that would abide by the government's strict censorship laws.
"Here, we address an underlying structural problem: now, we do not have the information required to make ethically-informed decisions about our work, our projects, and our employment", the letter reads.
Whether the company could or would launch search in China "is all very unclear", Pichai said, according to the transcript.
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Google didn't respond to requests for comment.
Google withdrew from China four years ago in protest at government censorship, which its co-founder Sergey Brin described as having the hallmarks of Soviet totalitarianism.
China has the world's largest Internet audience but has frustrated USA tech giants with content restrictions or outright blockages of services including Facebook and Instagram. After those discussions, a company official suggested changing the topic because the executives' comments were already being leaked online, one of the people said.
The China petition says employees are concerned the project, code named Dragonfly, "makes clear" that ethics principles Google issued during the drone debate "are not enough". They said the plans could be in violation the company's ethics code, which bars it from building or deploying technology that violates human rights.
The letter, first reported on by The New York Times earlier today, makes several demands for transparency, most bluntly stated as, "Google employees need to know what we're building".
"I genuinely do believe we have a positive impact when we engage around the world and I don't see any reason why that would be different in China", Pichai said.