The U.S. says it believes its ally Japan's allegation that China activated its weapons-guiding radar last month in an escalation of tensions between the two Asian powers in the East China Sea.

China denies Japanese claims that Chinese naval vessels locked their radar onto a Japanese destroyer and helicopter. It accuses Japan of fabricating reports to smear China.

But State Department spokeswoman Victoria Nuland said Monday that the U.S. was briefed by Japan, and quote, "we have satisfied ourselves that it does appear to have happened."

China and Japan are embroiled in a territorial spat over a cluster of tiny islands called Senkaku in Japanese and Diaoyu in Chinese. Washington says it takes no stance in the sovereignty dispute, but opposes "unilateral actions" undermining Japan's administration of the islands.