United Nations Secretary-General Antonio Guterres has praised a new treaty that outlaws nuclear weapons, calling it historic. The treaty – signed by 51 countries on Wednesday – is the first multilateral disarmament pact signed in over two decades.

Guterres said work had to be done to rid the world of its stockpile of 15,000 atomic warheads. “Today we rightfully celebrate a milestone,” he said. “Now, we must continue along the hard road towards the elimination of nuclear arsenals.”

The nine countries that possess nuclear weapons – the United States, Russia, Britain, China, France, India, Pakistan, North Korea and Israel – did not take part in the negotiations. The treaty will come into effect when 50 countries sign and ratify it.

Meanwhile, the United States, its North Atlantic Treaty Organisation allies, and other nuclear powers have strongly opposed the treaty. “At a time when the world needs to remain united in the face of growing threats, in particular the grave threat posed by North Korea’s nuclear programme, the treaty fails to take into account these urgent security challenges,” the 19-member Nato said.

The treaty was adopted by 122 countries at the United Nations in July after negotiations led by Austria, Brazil, Mexico, South Africa and New Zealand.