The world's nuclear powers continued to modernize and develop their arsenals amid a slight drop in the number of nuclear warheads, a Sweden-based peace research institute said Monday.

Global stockpiles of nuclear warheads numbered 13,865 at the beginning of this year, a decrease of 600 compared to early 2018, the Stockholm International Peace Research Institute (SIPRI) reported. Israel's stockpile was reported to be comprised of 80 to 90 warheads.

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The overall estimate included warheads that were active, in storage or ready to be dismantled.

"What we are reporting is that the overall number of nuclear weapons is in decline but all of the nuclear weapon-possessing states are either modernizing or have announced plans to modernize their forces," SIPRI researcher Shannon Kile told dpa.

Nine states are acknowledged to possess nuclear weapons: the United States, Russia, Britain, France, China, India, Pakistan, Israel and North Korea.

"It is clear that nuclear weapons remain a central element in their military strategies and national security doctrines," he added, citing that nuclear weapons "have not lost their perceived value for national security."

About 2,000 nuclear warheads were kept at a high state of readiness, SIPRI said.

The U.S. and Russia, the world's main nuclear powers, accounted for over 90 per cent of global stockpiles. The institute estimated that the U.S. had 6,185 warheads, while Russia had 6,500 as of January this year.

Both countries have reduced their stockpiles in line with a 2010 bilateral treaty on reducing nuclear arms known as New START. The reductions include obsolete warheads from the Cold War, but the pace of reductions is slowing compared to 10 years ago.

In addtion, there has been "no discussion about even doing a simple extension" of the New START treaty set to expire in 2021, Kile noted.

A continuing trend SIPRI highlighted was that the U.S. and Russia - as well as the other nuclear states - were engaged in modernizing their arsenals. According to the institute, they either have programs in operation or plan to develop new missile and aircraft delivery systems, and nuclear weapon production facilities.

The U.S., for instance, in 2018 announced plans to enhance the role of nuclear weapons in its military planning and operations.

"India and Pakistan have continued to both expand their nuclear arsenals," said Kile, who heads SIPRI's nuclear disarmament, arms control and non-proliferation program.

The two South Asian rivals have also expanded fissile material production capabilities, and could potentially "significantly expand the size of their nuclear arsenals in the next 10 to 15 years," he added.

Open gallery view A Pakistani-made Shaheen-III missile, that is capable of carrying nuclear warheads, is loaded on a trailer rolls down during a military parade to mark Pakistan National Day, in Islamabad, Pakistan, Ma Credit: Anjum Naveed/AP

India was estimated to have about 130 to 140 nuclear warheads, and Pakistan between 150 and 160, a slight increase on 2018.

The SIPRI report was based on open sources, including governments such as the U.S. and Britain, which are relatively transparent about their nuclear arsenals.

International pressure was needed to ensure that the nuclear weapon states provide more information about their stockpiles, but Kile said that was a challenging task as this was highly classified. Russia shares information with the U.S. under the terms of New START.

Britain was estimated to have 200 nuclear warheads, France 300, and China 290. Israel's stockpile comprised 80 to 90 warheads. North Korea was estimated to have 20 to 30 warheads but the information was extremely limited, according to SIPRI.

North Korea in 2018 continued "to prioritize its nuclear weapon program" even though it announced a moratorium on testing of nuclear weapons and medium- and long-range ballistic missiles, the SIPRI Yearbook said.

Kile said the moratorium was "a good sign, at least for the time being," but underlined it was not clear how long it will remain in place, and citing the need for the U.S. and North Korea to resume talks.