China said on Tuesday it will continue to “modernise” its nuclear arsenal and called upon the U.S. and Russia to reduce their own stockpiles a day after global powers pledged to prevent such weapons from spreading.

In a rare joint statement setting aside rising West-East tensions, the U.S., China, Russia, Britain and France reaffirmed their goal of creating a world free of atomic weapons and avoiding a nuclear conflict.

The five nuclear powers also committed to full future disarmament from atomic weapons, which have only been used in conflict in the U.S. bombings of Japan at the end of the Second World War.

But squaring that rhetoric with reality will not be easy at a time of spiralling tensions between those same global powers not seen since the Cold War.

There are growing global concerns about China’s rapid military modernisation, especially after its armed forces last year announced they had developed a hypersonic missile that can fly at five times the speed of sound.

The U.S. has also said China is expanding its nuclear arsenal with as many as 700 warheads by 2027 and possibly 1,000 by 2030.

On Tuesday, China defended its nuclear weapons policy and said Russia and the U.S. — by far the world’s largest nuclear powers — should make the first move on disarmament.

“The U.S. and Russia still possess 90% of the nuclear warheads on Earth,” Fu Cong, Director General of the department of arms control at the Chinese Foreign Ministry, said. “They must reduce their nuclear arsenal in an irreversible and legally binding manner.”

Mr. Fu dismissed U.S. claims that China was vastly increasing its nuclear capabilities. “China has always adopted the no first use policy and we maintain our nuclear capabilities at the minimal level required for our national security,” he said. But he said Beijing would “continue to modernise its nuclear arsenal for reliability and safety issues”.

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