The Chinese military would consider a pre-emptive nuclear strike if it had no other way to defend itself in a war against another nuclear-armed state, Kyodo News said Wednesday, citing Chinese documents.

The policy, called "Lowering the threshold of nuclear threats" may indicate a shift from China's pledge not to first fire nuclear weapons under any circumstances, the report said.

It may also fan concern in the United States, Japan and other regional powers, according to the Japanese news agency which obtained the internal documents.

The Chinese military's strategic missile forces, the Second Artillery Corps, would "adjust" its policy if another nuclear state conducts air strikes against Chinese targets "with absolutely superior conventional weapons," the document says, according to Kyodo.

China would first warn an adversary about a nuclear strike, but if the enemy attacks China with conventional forces, the Chinese military "must carefully consider" a pre-emptive nuclear strike, Kyodo said.

The documents suggest that the Second Artillery Corps educate its personnel in worst-case scenarios, Kyodo said, adding that it is rare for information on China's nuclear policy to come to light.

US military experts have argued since around 2007 that Beijing may have shown signs of altering its pledge of no first use of nuclear weapons, Kyodo said.

But in a sign of warming ties as the region contends with the threat of a nuclear-armed North Korea, US Defense Secretary Robert Gates is scheduled to arrive in China on January 9, the Pentagon announced in December.

The Pentagon said Gates will travel to China on the invitation of his Chinese counterpart, one year after Beijing broke off military relations with Washington in protest against a multibillion-dollar US arms package for rival Taiwan.