According to the Stockholm International Peace Research Institute (Sipri), China had 350 nuclear weapons in January 2021.
China said on Tuesday it will continue to "modernise its nuclear arsenal for reliability and security reasons" and called on Moscow and Washington to reduce their stockpiles of nuclear weapons.
Beijing's clarification comes after the five permanent members of the UN Security Council pledged the previous day to prevent the spread of nuclear weapons.
The five countries — the US, China, Russia, the UK and France — declared that "a nuclear war cannot be won" and promised to strengthen their measures to "prevent the unauthorised or unintended use of nuclear weapons".
According to the Stockholm International Peace Research Institute (Sipri), the United States had 5,550 nuclear weapons in January 2021, compared to China's 350.
Washington regularly accuses Beijing of rapidly building up its nuclear arsenal, but Fu Cong, director-general of the arms control department at the Chinese Foreign Ministry, said on Tuesday these were "false" claims.
"China has always adopted a no-first-use policy for nuclear weapons and we maintain our nuclear capabilities at the minimum level required for our national security," he told reporters.
"China will continue to modernise its nuclear arsenal for reliability and security reasons. Nuclear weapons are also getting older. We need to make sure that ours are up to date and can also serve as a deterrent."
Beijing regularly rejects Washington's invitations to join US-Russian talks on reducing their nuclear weapons, pointing out that its arsenal is much smaller.
"The US and Russia still possess 90% of the world's nuclear warheads. They must reduce their nuclear arsenal in an irreversible and legally binding way," Fu Cong said.