Former world leaders are urging their countries to join a UN treaty banning nuclear weapons.

The International Campaign to Abolish Nuclear Weapons, or ICAN, said on Sunday that 56 former leaders made the plea in an open letter.

The 56 include former prime ministers and foreign ministers from 20 NATO member states, as well as Japan’s former prime minister Hatoyama Yukio and former UN Secretary-General Ban Ki Moon from South Korea.

The letter says the risk of a nuclear weapon being used appears to be increasing. It blames that on “new types of nuclear weapons” and “the very real danger of cyber-attacks on nuclear infrastructure.”

It says “we appeal to current leaders to advance disarmament before it is too late.”

ICAN says the letter will be sent to the current leaders of the 20 NATO members, including Canada, Germany and Norway, along with Japan and South Korea.

Those countries have not joined the 2017 UN Treaty on the Prohibition of Nuclear Weapons. They rely on the nuclear umbrella of the United States.

The pact will take effect 90 days after the number of ratifications reaches 50. The figure now stands at 44.