U.N. Secretary-General António Guterres on Monday urged countries with nuclear weapons to stick to their pledge not to be the first to use them, warning that the nuclear arms race has resumed in the face of rising international tensions.

“This is the time … to call on countries with nuclear weapons to commit to the principle of not using them in the first place and to commit themselves not to use them and not to threaten non-nuclear countries,” Guterres told a news conference in Tokyo, two days after visiting Hiroshima to remember the victims of the atomic bomb detonated on August 6, 1945.

“I think no one can accept the idea that a new nuclear war could happen. This will be the destruction of the planet,” Guterres said. “What is clear is that if no one uses them for the first time, then there will be no nuclear war.”

Fears of a third atomic bomb have been growing in the face of Russian threats of a nuclear attack since their war in Ukraine began in February.

Moscow on Thursday attacked the Ukrainian city of Zaporizhia, which is home to Europe’s largest nuclear power plant. Asked about the attack, Guterres said “any attack on a nuclear power plant is suicidal.”

The Secretary-General expressed his full support for the International Atomic Energy Agency in its efforts to stabilize the plant and gain access to the compound to exercise its mandate.

Guterres said that after decades of nuclear disarmament efforts, the world is “going backwards” and noted that 13,000 nuclear bombs already exist, in addition to a huge investment to modernize atomic arsenals. “So it’s time to say, ‘Stop.'”

The billions of dollars spent on the arms race should be spent on other pressing issues, he said.

“The billions that are being used in this arms race must be used to combat climate change, fight poverty, address the needs of the international community,” he said.

The official announced that he would travel to Mongolia and South Korea to discuss ways to address North Korea’s nuclear development.

At a time when geopolitical tensions are rising and the nuclear threat is once again taking center stage, Japan’s strong and consistent defense for peace is more important than ever, Guterres said, urging Japan to use its unique position as the only country in the world to have suffered nuclear attacks to act as a “mediator to strengthen global cooperation and trust and solidarity.”

Guterres said he had Japan’s potential to lead the global fight against climate change, and specifically urged Tokyo to stop financing coal plants.

Japan, which has not clarified when it will completely ban coal-fired power plants, is seen as reluctant to commit to vetoing coal-fired power as soon as many European countries.

Current efforts in Japan, the world’s third-largest economy, are focused on developing methods to burn ammonia in conventional coal plants and gradually abandon coal use, perhaps by the 2040s. It also aims to promote “clean coal” technology in Asia to achieve emissions neutrality.

Critics and energy experts say Japan has overly ambitious targets for 20 to 22 percent of its electricity to come from nuclear power by 2030. By then, the country has promised to reduce its emissions to 46% of 2013 levels.

“There is no such thing as clean coal,” Guterres said. “For real change, I hope that Japanese public and private capital will stop financing coal altogether.”

Guterres said he hoped Japan, through multilateral development banks, would “immediately offer investments and support for developing countries to expand renewables and gain climate resilience” to seek solutions commensurate with their needs in the face of the climate emergency.

“I ask Japan to do the right thing, for Japan and for the world,” he said.