Gabbard did not specify her concerns, but Russian President Vladimir Putin has repeatedly brandished the specter of nuclear war as he cautions Europe and the United States against support for Ukraine.

Gabbard, a former congresswoman who has faced criticism in the past for her views on Russia, posted a video of grisly footage from the world's first nuclear attack and of her staring reflectively at the Hiroshima Peace Memorial.

On August 6, 1945, the United States obliterated Hiroshima, killing 140,000 in the explosion and by the end of the year from the uranium bomb's effects.

Three days later, a US plane dropped a plutonium bomb on Nagasaki, leaving around 74,000 people dead by the end of the year. Japan surrendered on August 15.

"This one bomb that caused so much destruction in Hiroshima was tiny compared to today's nuclear bombs," Gabbard said. "A single nuclear weapon today could kill millions in just minutes."

"As we stand here today closer to the brink of nuclear annihilation than ever before, political elites and warmongers are carelessly fomenting fear and tensions between nuclear powers," she said.

"Perhaps it's because they are confident that they will have access to nuclear shelters for themselves and for their families that regular people won't have access to."

Taking a tone more customary for a politician or activist than the director of national intelligence, Gabbard said: "So it's up to us, the people, to speak up and demand an end to this madness."

Japanese media reports said the comments were "extremely rare" for an incumbent US government official, and at odds with Washington's past justification of the bombings.

Yoshimasa Hayashi, Japan's top government spokesman, declined to comment directly on Gabbard's video.

But he said an "accurate understanding" of the destruction and suffering caused by atomic bombs would "serve as the basis for various efforts toward nuclear disarmament".

"It's important for Japan to continue its realistic, pragmatic efforts with the United States to realise a nuclear-free world, based on the belief that the carnage in Hiroshima and Nagasaki must not be repeated," Hayashi said.

Gabbard's remarks come as aides to President Donald Trump voice growing frustration with Putin, who has refused US-led, Ukraine-backed calls for a temporary ceasefire.

Secretary of State Marco Rubio, whom Gabbard criticized before the two entered Trump's cabinet, has warned that the United States could walk away from diplomacy over the Ukraine conflict if there are no positive signs.

Gabbard, a former Democrat, faced a heated confirmation hearing but ultimately prevailed after Democrats and some Republicans questioned her past statements, including some supportive of Russian positions.

She has said that the European Union and Washington should have listened to Russian security concerns about Ukraine joining NATO.

Gabbard's visit to Hiroshima comes ahead of the 80th anniversary of the world's only atomic bombings.

The United States has never apologized for the attacks.