Even with international agreements to prioritize nuclear disarmament, all nine nuclear-armed countries continue to either develop new nuclear weapon systems or upgrade their existing ones, according to a new report from the Stockholm International Peace Research Institute (SIPRI).

"Despite renewed international interest in prioritizing nuclear disarmament, the modernization programmes under way in the nuclear weapon-possessing states suggest that none of them will give up their nuclear arsenals in the foreseeable future," Senior SIPRI Researcher Shannon Kile said in a statement.

The countries - the United States, Russia, the United Kingdom, France, China, India, Pakistan, Israel and North Korea - possessed nearly 15,850 nuclear weapons at the start of 2015. Of those, 4,300 were deployed with operational forces, and 1,800 were kept in a state of high operational alert, SIPRI said.

The U.S. and Russia, the world's two largest nuclear powers, accounted for 90 percent of all nuclear weapons, the U.S. having an estimated 7,260 warheads and Russia having 7,500. That's down from a total of 7,300 and 8,000 in 2014, respectively. "Both countries have extensive and expensive long-term modernization programmes under way for their remaining nuclear delivery systems, warheads and production," the report said.

The Obama administration announced in September that it plans to spend $1 trillion over the next 30 years to upgrade its aging nuclear weapons systems, reported The New York Times.

While the number of nuclear warheads in the world is declining, led largely by the efforts of the U.S. and Russia, it's declining at a slower pace than a decade ago, and all "are either developing or deploying new nuclear weapon systems or have announced their intention to do so," SIPRI said.

In the case of China, SIPRI said, "this may involve a modest increase in the size of its nuclear arsenal," and India and Pakistan are "both expanding their nuclear weapon production capabilities and developing new missile delivery systems."

It's harder to assess North Korea's program, but the isolated nation is believed to be advancing its nuclear program and is estimated by SIPRI to have six to eight nuclear devices. Pyongyang is thought to already have at least 10 nuclear weapons and as many as 20, with 40 expected to be developed by 2020, the Wall Street Journal recently reported. 

SIPRI based its data on open sources, including the U.S. and British governments, which are thought to be transparent about their arsenals. SIPRI gave the U.S. the highest mark for transparency, while it said Britain and France were more restrictive, and Russia didn't officially divulge anything, except in bilateral contracts with the U.S. China also revealed little about its arsenal, and the only information for India and Pakistan were missile test announcements.