In total, the world's nine nuclear-armed states possessed 12,187 warheads combined at the start of this year, according to the annual Nuclear Weapons Ban Monitor report.

That marks a dramatic decrease from the peak of more than 70,000 such weapons in the mid-1980s, during the Cold War, and a reduction of 144 weapons from early 2025.

But at the same time, the number of nuclear weapons quickly available for use has been steadily rising around the world in recent years, hitting an estimated 9,745 last year, said the monitor.

The report, published by the Norwegian People's Aid (NPA) humanitarian organisation in cooperation with the Federation of American Scientists, said that marked an increase of 141 warheads from 2024.

It meant that the world currently has at its fingertips the equivalent of 135,000 Hiroshima-sized bombs -- just one of which killed 140,000 people in 1945, it said.

And 40 percent of those warheads, 4,012 of them, had been deployed on ballistic missiles in silos, on mobile launchers, submarines or bomber bases last year, said the report, marking a hike of 108 from 2024.

- 'Dangerous' -

"The continued annual rise in deployed warheads is a concerning development, increasing the risks of rapid escalation, miscalculation, and accidental use," said Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists and one of the main contributors to the report.

"This make the world more dangerous for us all," he pointed out in a statement.

The monitor said the development was all the more concerning against a backdrop of escalating conflicts in Europe, Asia and the Middle East involving nuclear-armed states.

It also highlighted "the erosion of the long-standing disarmament, non-proliferation, and arms control regime", including the lapsing last month of New START -- the last treaty between top nuclear powers Russia and the United States.

"What we are witnessing is more than a new arms race," NPA chief Raymond Johansen warned in the statement.

"It is a reversal of hard-won constraints on nuclear dangers."

The monitor detailed how the world was pulling in opposite directions on the nuclear issue, with a growing number of countries signing on to efforts towards a total ban on all atomic weapons.

By the end of 2025, 99 countries had joined the Treaty on the Prohibition of Nuclear Weapons (TPNW), either as parties or signatories, which was negotiated at the UN in 2017.

Meanwhile the nuclear-armed states -- none of which have joined the treaty -- are investing heavily in modernising and expanding their arsenals.

And 33 so-called "umbrella" states "actively support and reinforce these policies", the statement pointed out.

In all, 47 countries actively oppose the TPNW, with three-quarters of them in Europe, it said.

"The states that claim nuclear weapons give them security, particularly in Europe, need to realise there is no shelter to be had under a nuclear umbrella," said Melissa Parke, head of the Geneva-based International Campaign to Abolish Nuclear Weapons, which won the 2017 Nobel Peace Prize for its work championing the treaty.

"They must join the global majority supporting total nuclear disarmament," she said in the statement.