Beijing on Monday blasted US lawmakers' legislation amendment that would reaffirm Washington's military support to Japan in the heated territorial row over China's Diaoyu Islands.
Observers warned that the US Senate's signal of support for Japan will further fuel Tokyo's confrontational attitude toward Beijing in the territorial fray, which has hurt Sino-Japanese ties in the past few months, and that the danger of the crisis spiraling out of control is mounting.
The amendment ― attached to the US National Defense Authorization Act for Fiscal Year 2013 and passed last week ― said that while the US "takes no position" on the sovereignty of the islands, it "acknowledges the administration of Japan" over the islands.
It added that "unilateral actions of a third party" would not affect its position.
The US Senate is still debating the full bill. The Senate and US House both must pass the legislation and President Barack Obama must sign it before it becomes law.
Chinese Foreign Ministry on Monday said Beijing "expresses serious concern and firm opposition" to the amendment.
The amendment reaffirms the US commitment to Japan under the Treaty of Mutual Cooperation and Security and warned that an armed attack against either party "in the territories under the administration of Japan" would be met in accordance with its provisions.
"The US-Japan security treaty is a product of the Cold War and should not go beyond the bilateral scope or undermine the interests of a third party," Foreign Ministry spokesman Hong Lei told a daily news conference in Beijing.
The US has previously vowed that the country will not "take sides" in the territorial row between China and Japan, yet Washington has also reiterated that the islands fall within the scope of the US-Japan treaty, which was signed in 1960.
The bill shows Washington's actual partiality behind its "seemingly neutral posture", said Jin Canrong, an American studies professor and deputy dean at the School of International Studies at Renmin University of China in Beijing.
"The new amendment is casting a huge shadow over the bilateral efforts from Beijing and Tokyo to rein in the risk of a major conflict over the Diaoyu Islands," Jin said.
The hostile move shows Washington's anxiety over China's rise, which is not facilitating Sino-US ties, Jin added.
Said Hong: "We hope the US will bear in mind the broader interests of peace and stability in the region, honor its words with actions, and refrain from sending self-contradictory and wrong signals".
Feng Wei, an expert on Japanese studies at Fudan University in Shanghai, said China's growing presence in the region is viewed as a threat to Japan, a major ally of the US.
"Tokyo's top policymakers need clarified support from Washington," Feng said.
The Diaoyu Islands and affiliated islets have been China's inherent territory since ancient times, yet Japan stole the islands at the end of the 1894-95 Sino-Japanese War. According to key post-war legal documents, including the Declaration of Cairo, the islands have been returned to China.
The Japanese government in mid-September made an illegal "purchase" of part of the islands from a so-called private owner, a move that moved the country's ties with China into a tailspin and prompted Beijing's strong countermeasures to assert sovereignty.
Recently, "the US-Japan alliance" has been a catchphrase within Japan from the mouths of forerunners for the upcoming general election on Dec 16, and many politicians believe seeking a boost to the alliance will help Japan in the dispute with China.
The Japanese newspaper Mainichi Shimbun said the top three parties ― the main opposition Liberal Democratic Party, the ruling Democratic Party of Japan and the Japan Restoration Party ― all directly mentioned the Japan-US alliance in their campaign platforms when elaborating on their diplomatic and security proposals.
Washington lawmakers' latest signal has given a boost to former Japanese prime minister Shinzo Abe, the hard-line president of the LDP, who is favored to win the election, Feng Wei said.
AFP contributed to this story.