"As Communist China moves to at least double the size of its nuclear stockpile, modernizing our nuclear force and maintaining readiness is essential to a free and open Indo-Pacific", Esper tweeted.
Esper has made his remarks following a meeting with Japanese Defence Minister Taro Kono. After the meeting, the US Defence Ministry said that both countries were concerned "with Chinese actions in the region".
On 22 June, Russia and the United States launched talks in Vienna in an effort to save the New Strategic Arms Reduction Treaty (New START), the last agreement limiting the two countries' nuclear arsenals that is set to expire in February. China has repeatedly declined the invitation to join the talks, dashing US hopes of making them trilateral.