Beijing on Sunday blasted Washington's latest intervention in the Diaoyu Islands issue, warning the US to be "responsible" and "choose its words carefully".

The warning came as Beijing prepared to receive a high-ranking member of the governing coalition in Tokyo who will reportedly deliver a letter "aimed at improving ties''.

But sending messengers to Beijing will be of little value, observers warned, if Washington's support for Tokyo's stance risks the situation getting out of control. US Secretary of State Hillary Clinton on Friday revealed Washington's lack of neutrality when she said the islands were "under the administration of Japan", a phrase that China rejects.

Clinton told reporters after meeting with visiting Japanese Foreign Minister Fumio Kishida that the US opposes "any unilateral actions" that would seek to undermine Japan's "administration", a remark observers see as clearly backing Japan.

Dong Manyuan, vice-president of the China Institute of International Studies, said Clinton's remarks show an evident preference for Japan's stance, and this runs the risk of bolstering far-right nationalists in Tokyo.

The comments are "ignorant of facts and indiscriminate of rights and wrongs", and Washington cannot deny its historical responsibility on the issue of the islands, Foreign Ministry spokesman Qin Gang told reporters on Sunday.

"Tokyo may feel less inhibited in carrying out further provocation, and the situation will become more unstable," Dong warned. "A major conflict breaking out over the islands, along with further confrontation between China and the US, does not serve US national interests."

Yet Dong added that as "Clinton is retiring soon, it remains to be seen if the incoming secretary of state, John Kerry, will follow the existing policies".

The Diaoyu Islands and affiliated islets have belonged to China for centuries, but Japan illegally grabbed the islands at the end of the 1894-95 Sino-Japanese War. Key wartime documents, including the Cairo Declaration, covered the return of the islands.

Yet, despite opposition from China, the US and Japan cut a backroom deal in 1971 giving Japan so-called "administration" over the islands.

Beijing urges the US to be "responsible" and careful with its words, Qin said.

Dong Manyuan said that perhaps Washington is also privately reining in Tokyo to prevent a serious escalation.