Sen. Jeff Merkley (D-Ore.) said Thursday that he has little confidence Donald Trump will act rationally on nuclear weapons as commander in chief, in part because he thinks the president-elect behaves like a child.

Responding to Trump’s vague tweet Thursday morning stating that he wants to “strengthen and expand” the nuclear capability of the United States, Merkley told MSNBC’s Chris Hayes that the apparent escalation of tensions terrified him.

“We have an incoming president who has kind of the maturity of a five-year-old, wrapped by a massive ego,” Merkley said. “And to have that just a second away from a nuclear trigger is very, very scary.”

Trump’s tweet came just hours after Russian President Vladimir Putin issued a similar call to strengthen his nation’s nuclear arsenal. Experts have expressed concerns that this sort of rhetoric could signal a departure from decades of cooperation against nuclear proliferation in the international community.

“This is an area where it’s constrained by international agreements, as it should be because of the risk of nuclear weapons,” Merkley said.

Although Trump’s tweet left many people worried, it also sparked uncertainty within his own camp, as his surrogates have repeatedly sought to walk back or clarify the president-elect’s position on nukes.

On Thursday, Trump’s aides claimed their boss wasn’t actually arguing for building more nuclear weapons ― to which the president-elect reportedly responded Friday that he was fine with doing just that.

“Let it be an arms race,” he allegedly told MSNBC’s Mika Brzezinski. “We will outmatch them at every pass and outlast them all.”

Trump’s incoming press secretary Sean Spicer later tried to temper those remarks, stating flatly that “there is not going to be” an arms race.

This confusion isn’t likely to reassure people like Merkley, who believes nuclear policy should be approached with a level head and an understanding of the extreme stakes at play.

“There is really so much at risk here of irrational action,” the senator told Chris Hayes on Thursday.

You can watch the full interview above. Merkley’s comments on nukes and Trump’s maturity begin around 7:20.