A UN-backed monitoring group urged the United States and eight other countries on Friday to ratify a worldwide ban on atomic test blasts ahead of the International Day against Nuclear Tests this weekend.

"Now is the time for the nine states whose ratification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) will bring it into force to show the political will and fully endorse it," said the head of the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), Tibor Toth.

The CTBT, which bans nuclear blasts for military or civilian purposes, was drawn up in 1996 and has so far been signed by a total 182 countries and ratified by 153.

But nine key states, namely China, North Korea, Egypt, India, Indonesia Iran, Israel, Pakistan and the United States, still need to ratify it before it can come into force.

US President Barack Obama has said that Washington is committed to ratifying the CTBT. But it seems likely to hold off doing so until after the new START or Strategic Arms Reduction Treaty has first been cleared by the US Senate.

Last year, the UN General Assembly declared August 29 as the International Day against Nuclear Tests.

The date was chosen because August 29, 1949, was when the then Soviet Union followed the US and detonated its first nuclear device, effectively starting the nuclear arms race.

The site of the first Russian test was at Semipalatinsk in Kazakhstan and a total of 450 bombs were tested there until 1991, when Kazakh President Nursultan Nazarbayev ordered the closure of the site, also symbolically on August 29.

"The declaration of 29 August as the International Day against Nuclear Tests is an acknowledgement of the need to halt nuclear testing once and for all," CTBTO chief Toth said.

"The will to pursue a nuclear-weapon-free world is not in short measure but we need to observe August 29 as a time to act and not to wait," he said.

"The hands of states seeking to develop nuclear weapons and the hands of those that already have them will be tied without their ability to test," the CTBTO chief argued.