SEOUL—North Korea’s state media described President Donald Trump’s “America First” policy as “Nazism in the 21st century,” and compared the U.S. president to Adolf Hitler, in the harshest language that Pyongyang has directed at the Trump administration.

Mr. Trump’s policy “is the American version of Nazism far surpassing the fascism in the last century in its ferocious, brutal and chauvinistic nature,” Pyongyang’s state-controlled Korean Central News Agency said in a report published Tuesday.

The language reflects North Korea’s hardening line on Mr. Trump, as the U.S. president turns up the heat on the regime nearly six months into his administration. While North Korean propaganda regularly targets the U.S. with military threats, it has remained quiet until recently on the president himself.

Typically, North Korea has given new leaders in Washington and Seoul a wider berth at the beginning of their terms, as Pyongyang feels out their likely policies. Relations with the Obama administration soured in 2012 after a rapprochement between the two countries ended and North Korea ramped up its rhetoric against the president.

Shortly after Mr. Trump took office in January, the U.S. government approved visa applications for Choe Son Hui, a top North Korean diplomat, to visit New York for back-channel talks, signaling a possible willingness to engage in dialogue. But the State Department eventually nixed the meeting following the assassination of Kim Jong Nam, the half-brother of North Korean dictator Kim Jong Un, in Malaysia.

Since then, the U.S. military has conducted several flyovers near the Korean Peninsula using B-1B bombers and directed a Navy aircraft carrier group to the region—all to North Korea’s consternation.

In April, U.S. Secretary of State Rex Tillerson called for more diplomatic isolation of North Korea.

“The world has never seen peace since the emergence of the American-first principle,” the KCNA report said Tuesday.

The U.S. Embassy in Seoul declined to comment on the remarks.

Last month, Mr. Trump said in an interview with Bloomberg News that he would be honored to meet Kim Jong Un “under the right circumstances.”

Robert Carlin, a visiting scholar at Stanford University’s Center for International Security and Cooperation, said the harsher rhetoric from Pyongyang, including recent articles in the North Korea’s party mouthpiece calling Mr. Trump a “reckless war maniac” and a “lunatic,” comes after several months in which North Korea refrained from making personal attacks on Mr. Trump.

“The coarsening language toward the administration, and toward the president himself, seems to reflect a slowly sharpening discussion within the regime,” Mr. Carlin, a former longtime Central Intelligence Agency analyst, wrote in an analysis for 38 North, a North Korea-focused blog.

In Tuesday’s article, the North complained about sanctions on the country, which KCNA described as “an unethical and inhumane act, far exceeding the degree of Hitler’s blockade of Leningrad.”

The article also described Mr. Trump’s decision to withdraw the U.S. from the 2015 Paris climate-change agreement as a violation of international norms that surpassed the horror of the Nazis’ concentration camps. Mr. Trump has called his decision to withdraw from the pact a “reassertion of our sovereignty.”

Satellite imagery shows that North Korea operates a network of prison camps, which a United Nations report in 2014 compared with “the camps of totalitarian states of the twentieth century.” North Korea denies their existence.

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