"The renewed focus on the strategic importance of nuclear deterrence and capacity is a very worrying trend," the Stockholm International Peace Research Institute (SIPRI) said.

"The world needs a clear commitment from the nuclear weapon states to an effective, legally binding process towards nuclear disarmament," SIPRI head Jan Eliasson said.

SIPRI, a well respected authority, said nine countries -- the United States, Russia, Britain, France, China, India, Pakistan, Israel and North Korea -- had 14,465 nuclear warheads at the beginning of this year, of which 3,750 were actually deployed.

This compared with 14,935 warheads at the start of 2017, SIPRI said, with the reduction largely due to the United States and Russia as they fulfill arms control commitments agreed in their 2010 START treaty.

The two countries between them hold more some 92 percent of total warheads, SIPRI noted, and they both have long-term modernisation programmes in place.

Russia in particular has in recent years re-stated the importance of nuclear weapons to its strategic defence, sparking concerns in NATO that Moscow might be more willing to use them in a crisis.

At the same time, Britain with 215 warheads, France 300, China 280, India 130-140, Pakistan 140-150, Israel 80 and North Korea 10-20, were all either deploying or planning to deploy new nuclear weapons system, it said.

"North Korea also demonstrated unexpected rapid progress in the testing of two new types of long-range ballistic missile delivery systems," it noted.