UNITED NATIONS -- United Nations (UN) Secretary-General Antonio Guterres on Thursday called for progress toward a world free of nuclear weapons on the occasion of the 76th anniversary of the atomic bombing of the Japanese city of Hiroshima.

"I am deeply concerned by the lack of progress toward the goal of a nuclear-free world," he said in a video message for a ceremony to mark the anniversary.

States in possession of nuclear weapons have been modernizing their arsenals in recent years, sparking a new arms race, but the decisions by Russia and the United States to extend the New Strategic Arms Reduction Treaty and engage in a dialogue on arms control "are welcome first steps towards reducing the risk of nuclear catastrophe," he said.

"I call on all states that possess nuclear weapons to adopt risk reduction measures, individually and jointly," said Guterres. "The only guarantee against the use of nuclear weapons is their total elimination."

The United States dropped an atomic bomb on Hiroshima on Aug. 6, 1945, and a second on Nagasaki three days later, killing about 210,000 people by the end of that year. The nuclear attacks eventually brought about the end of Japan's colonization and invasion of many East Asian countries, and the end of World War II.