U.S. President Donald Trump’s readiness to threaten the use of America’s nuclear arsenal has unnerved diplomats and academics and inflamed tensions with North Korea over the first year of his presidency. Now former nuclear missile launch operators are adding themselves to the list of concerned observers.

In an open letter to Congress on Thursday, a group of 17 former nuclear launch officers argued that President Trump “poses a clear and present danger to the country and the world” and warned that “there are no reliable safeguards” to prevent Trump from deciding to launch a nuclear missile on his own.

The letter was published by international nuclear disarmament advocacy group Global Zero, and follows a similar letter written at the close of the 2016 presidential election.

Trump’s “inflammatory rhetoric has put the United States on a collision course with North Korea,” the new letter says, citing a recent exchange in which Trump boasted that his “nuclear button” is “bigger and more powerful” than North Korea’s.

“Threats of ‘fire and fury’ and total destruction of the Kim dictatorship undercut diplomatic efforts and increase the likelihood of stumbling into conflict,” the officers wrote.

Read more: Former Nuclear Launch Officers Pen Letter Against Donald Trump

Although there is no literal “nuclear button,” the president does hold a the unique and singular authority to order a nuclear strike. The officers mention several “common-sense” proposals currently in Congress that would limit Trump’s ability to unilaterally deploy a nuclear weapon without the approval of the Defense Secretary, Attorney General, or Congress.

In Nov., a rare Senate Foreign Relations Committee discussed the nuclear chain of command after Trump warned North Korea that any attack would be “met with fire and fury like the world has never seen.”

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Write to Eli Meixler at eli.meixler@time.com.