An open letter from 1,367 researchers and engineers at frontier AI labs, mainly OpenAI, Anthropic and Google Deepmind, warns that "there is a real risk that capability development rapidly accelerates beyond our ability to understand or control the resulting systems." The letter asks the US government to support "an international effort to develop the technical and governance tools needed to deliberately pace the frontier of automated AI development." Signers include CEOs, co-founders, chief scientists and hundreds of well-known AI researchers. The letter follows months of concerns about "recursive self-improvement" or RSI, discussed at the "Recursive" conference in mid-May in San Francisco and in Anthropic's RSI warning, "When AI builds itself", in early June. Broader concerns also motivated the formation of the Coalition of Concerned AI Staff, linking employees of all major AI companies. The letter includes personal statements from almost a hundred signers, with at least three major themes emerging. Pacing the frontier would require a licensing process with strong verification of safety, registration of AI systems with the ability to monitor and terminate them, and enforceable international agreements. AI systems escaping their confines and breaking into other companies' computers have become an almost daily occurrence, serving as an early warning.