By Patricia Zengerle
WASHINGTON, Dec 5 (Reuters) - The Democratic and Republican leaders of the U.S. Senate Foreign Relations Committee urged China on Thursday not to implement an air defense zone over disputed islands in the East China Sea, saying in a letter that Beijing's recent activities threatened "vital national interests of the United States."
Beijing's decision to declare the air defense identification zone in an area that includes disputed islands has triggered protests from the United States, Japan and South Korea, and was a central topic of discussions during a visit to Japan, China and South Korea this week by U.S. Vice President Joe Biden.
President Barack Obama's administration has made clear it will stand by treaty obligations that require it to defend the Japanese-controlled islands, but is also reluctant to get dragged into any military clash between rivals Japan and China.
The four senators' letter similarly urged China not to implement the zone as announced and urged Beijing to engage in diplomatic dialogue to address disputes in the South and East China Seas.
"We view this unilateral action as an ill-conceived attempt to alter the status quo, increasing the possibility of misunderstanding or miscalculation," Senators Robert Menendez and Bob Corker said in a letter that was also signed by Democrat Ben Cardin and Republican Marco Rubio, two other committee members.
Menendez is the chairman of the foreign relations panel and Corker its top Republican.
The letter to Cui Tiankai, China's ambassador to the United States, referred to a "disturbing trend of increasingly hostile Chinese maritime activities," including incursions by its ships into the waters of Japan, the Philippines and Vietnam.
"These actions threaten freedom of air and maritime navigation, which are vital national interests of the United States," the senators wrote.
Under the zone's rules, all aircraft have to report flight plans to Chinese authorities, maintain radio contact and reply promptly to identification inquiries. U.S., Japanese and South Korean military aircraft have breached the zone without informing Beijing since it was announced on Nov. 23.
Japanese and South Korean commercial carriers have been told by their governments to ignore the rules. Three U.S. airlines, acting on government advice, are notifying China of plans to transit the zone.
China has repeatedly said the zone was designed to reduce the risk of misunderstandings, and stressed that since it was set up there had been no issues with freedom of flight for civilian airlines. (Reporting by Patricia Zengerle; Editing by Ken Wills)