The New START treaty ended with the turn of the calendar to February 5, after US President Donald Trump did not follow up on Russian counterpart Vladimir Putin's proposal to extend warhead limits in the agreement for one year.

Terumi Tanaka, co-chair of Nihon Hidankyo, a group of survivors of the 1945 US nuclear bombings of Hiroshima and Nagasaki, said the world has failed to see the urgency of the issue.

The staunchly pacifist grassroots group received the Nobel Peace Prize in 2024.

"Given the current situation, I have a feeling that in the not-too-distant future, we'll actually have a nuclear war and head toward destruction," Tanaka said at a press conference held with fellow Japanese campaigners.

The 93-year-old said he feared that citizens of nuclear-armed nations may not give much thought to their country's weapons.

"They might even see it as proof that they're a great power. That's a huge mistake," he warned.

The United States dropped atomic bombs on Hiroshima and Nagasaki, where Tanaka lived, in August 1945. Shortly afterwards, Japan surrendered, ending World War II.

Around 140,000 people died in Hiroshima and about 74,000 others in Nagasaki, including many from the effects of radiation exposure.

It was the only time that atomic weapons were used in warfare.

Campaigners around the world have warned that the end of the New START treaty could unleash a new arms race between the world's top nuclear powers and encourage China to expand its arsenal.

Washington has said a new agreement would have to include China.

But campaigners said that Tokyo has failed to take any effective steps to encourage arms control in East Asia or to engage China to join such efforts.

"I think there is absolutely no effort to realise dialogue with China," said Hideo Asano, activist at Japan Campaign to Abolish Nuclear Weapons.

The Japanese government said it would "continue working closely with the United States" to build a control framework for nuclear weapons.

"Our country has consistently considered it important to pursue arms control and disarmament efforts that firmly involve relevant nations, including the United States, Russia, and China," Deputy Chief Cabinet Secretary Kei Sato said at a press conference.