China said on Sunday it is "firmly opposed" to the United States' new nuclear weapons policy statement, describing its speculation about Chinese intentions as "wild guesses".

The US Defense Department's Nuclear Posture Review released on Friday outlines the Pentagon's nuclear ambitions under President Donald Trump, while spelling out how it foresees nuclear threats in the coming decades.

Although the review largely focuses on Russia, several sections are dedicated to the lack of transparency in China's nuclear buildup.

The report says China has added new types of nuclear capabilities — ranging from a mobile intercontinental ballistic missile to a new ballistic missile submarine — "with little to no transparency into its intentions".

The report makes "wild guesses" about China's intentions and exaggerates the threat of its nuclear force, defence ministry spokesman Ren Guoqiang said in a statement.

China is "firmly opposed to this", Ren said.

China "has always kept its own nuclear forces at the minimum level required by national security", Ren said, pointing out that the US has the world's largest nuclear arsenal. "We hope that the United States will abandon its Cold War mentality."

Since taking office in 2012 President Xi Jinping has pushed for a muscular China, including calls last October to develop a "world-class" military by 2050.

China's neighbours have watched warily as the People's Liberation Army has upgraded its arsenal with increasingly sophisticated weaponry and sought to create a more effective and professional fighting force.

Upgrades to its nuclear arsenal have received less attention, partly because of its small size, estimated by the Stockholm International Peace Research Institute at just 270 warheads compared to 6,800 for the US.

China has also long maintained it will never allow first use of its atomic weapons.

China "always abides by the principle of no first use of nuclear weapons under any circumstances" and will "unconditionally not use or threaten to use nuclear weapons against non-nuclear weapon states", Ren said.

US to develop tactical weapons

The US military wants to revamp its nuclear arsenal and develop new low-yield atomic weapons, largely in response to Russian actions in recent years, the Pentagon said.

Moscow on Saturday denounced the "bellicose" and "anti-Russian" nature of the new US nuclear policy, warning it would take necessary measures to ensure its own security.

“From first reading, the confrontational and anti-Russian character of this document leaps out at you,” Russian Foreign Minister Sergei Lavrov said on Saturday in a statement.

Speaking on the new policy, US Undersecretary of State for Political Affairs Thomas Shannon had said: “Important to this deterrence is maintaining our capabilities so that the United States can respond decisively across the full spectrum of potential nuclear and non-nuclear scenarios."

He did not rule out the possibility of a US response to a nuclear threat, but said that “the United States will only consider the use of nuclear weapons in extreme circumstances to defend the vital interests of the United States, its allies and partners”.

In a statement issued by his office on Friday evening, US President Donald Trump had said the new strategy also “commits to improving efforts to prevent, detect, and respond to nuclear terrorism”.

Shannon had further explained the US position on this issue at a Pentagon news briefing, saying that the new strategy “makes clear that the United States will hold accountable any state or non-state actor that supports terrorist efforts to obtain or employ a nuclear weapon”.

The new strategy also commits the US to developing a new batch of smaller nuclear weapons, arguing that these tactical weapons will work as a greater deterrent because enemies might think the US might actually use them.