New York, Oct 30 : President Donald Trump has announced that the US will start testing nuclear weapons 'immediately' in a break from 33 years of the country's self-restraint.

Hours before his meeting with China's President Xi Jinping in South Korea on Thursday, he said in a cryptic post on Truth Social, 'Because of other countries' testing programs, I have instructed the Department of War to start testing our Nuclear Weapons on an equal basis'.

'That process will begin immediately', he added.

Trump said that the US had more nuclear weapons than any other country and this was due to "a complete update and renovation of existing weapons', during his first term.

That was accomplished without any nuclear tests, which scientists say are not necessary anymore because computer simulations can do the job.

Trump wrote that he had 'no choice' but to expand the nuclear arsenal although he hated it, because 'Russia is second, and China is a distant third, but will be even within 5 years'.

Elements in his post were unclear. For example, he said he was ordering the nuclear tests 'because of other countries' testing programmes'.

Only North Korea, across from where he made the post, has carried out a nuclear test in recent years, and that was in 2017.

There have been no other tests since 1998, when India and Pakistan set off test blasts.

The last nuclear weapon test by the US was in 1992, by China in 1996 and by Russia's predecessor, the Soviet Union, in 1990.

NATO allies France's last test was in 1996, and Britain's was in 1991 at the US facility in Nevada.

Both India and Pakistan had their last nuclear tests in 1998.

What provoked Trump to order the tests may have been Russia's President Vladimir Putin's boasts this week that his country had successfully tested two nuclear-powered systems.

On Wednesday, he said his country had tested a nuclear-powered underwater drone capable of reaching the US west coast the previous day.

There were 'no ways to intercept' the drone named Posiedon, he said.

It is reported to be able to cause a tsunami effect to engulf its target.

'There is nothing like this in the world in terms of the speed and the depth of the movement of this unmanned vehicle -- and it is unlikely there ever will be', Putin said.

And on Sunday, Putin said a nuclear-powered cruise missile was tested. But they were not nuclear weapons, but systems to deliver them.

Trump could also have meant delivery systems when he said 'weapons' because nuclear bombs are built and tested by the Department of Energy, not by the Department of War, which controls the delivery systems.

If Trump wants to carry out a test, he can, because the US is not a signatory to the Comprehensive Nuclear Test Ban Treaty; moreover, it has not come into effect, lacking the signatures of countries with nuclear weapons, including India.

A pact between Russia and US to limit nuclear weapons-armed intercontinental missiles launched from sea or land to 700 each and the deployed weapons to 1,550 is to expire on February 5.

Known as the New START, it replaced the earlier Strategic Arms Reduction Treaties (START) and while limiting the deployed missiles, it did not cap the bombs.

Putin has suggested continuing with the limits for another year while negotiating a new treaty.

China, which is on a tear to match the US and Russia, was not a part of New START.

/IANS