As of the beginning of 2015, nine countries together possess about 15,850 nuclear weapons, with 4,300 deployed with operational forces, the Stockholm International Peace Research Institute said Tuesday.

They are five members of the Nonproliferation Treaty - the U.S., Russia, China, the U.K. and France - plus four other nations that are not legally recognized nuclear weapons states: India, Pakistan, Israel and North Korea.

The total number of warheads is declining around the globe chiefly because Washington and Moscow are gradually scaling back their nuclear arsenals. But both old Cold War foes are running extensive and costly long-term modernization programs for their remaining delivery systems, warheads and production.