China's new maritime air defense zone unenforceable, a Japanese government spokesman said Monday, in a continuing war of words over waters that include islands claimed by both.

Deputy Chief Cabinet Secretary Katsunobu Kato told a daily news conference that China's declaration of the sea air defense zone alters the state of affairs in the East China Sea and escalates a tense situation.

"It can invite an unexpected occurrence and it is a very dangerous thing as well," he said.

"The step just announced unjustly interferes with the freedom to fly above the open sea, which is a general principle under the international law," he said. "The measure is not enforceable to our country."

He said Japan had demanded that China withdraw all the steps that would "interfere with the freedom to fly over public sea."

Earlier Monday, China's Foreign Ministry said it has complained to the United States over its "irresponsible remarks" about China's drawing up of the zone for the disputed islands, which are administered by Japan.

China's Defense Ministry also called Japan's objections to its East China Sea Air Defense Identification Zone "absolutely groundless and unacceptable" and said it had made solemn representations to the Japanese Embassy in Beijing.

On Saturday, Beijing issued a map of the zone and a set of rules, which say all aircraft must notify Chinese authorities and are subject to emergency military measures if they do not identify themselves or obey Beijing's orders.

Foreign Ministry spokesman Qin Gang said in a statement Sunday that the zone's aim is to defend China's sovereignty and the security of its airspace and land. He said it is not aimed at any country and it does not affect freedom of overflight.

U.S. Secretary of State John Kerry and Secretary of Defense Chuck Hagel have both said the U.S. is "deeply concerned" about China's unilateral action.

"This unilateral action constitutes an attempt to change the status quo in the East China Sea," Kerry said in a statement released Saturday. "Escalatory action will only increase tensions in the region and create risks of an incident."

Qin said China made solemn representations Sunday to U.S. Ambassador Gary Locke for the U.S. "to correct its mistakes and stop making irresponsible remarks on China."

Defense Ministry spokesman Yang Yujun said the ministry had complained to the U.S. Embassy's military attache on Sunday evening.

The U.S. doesn't take a position on who has sovereignty over the islands — called Diaoyu in Chinese and Senkaku in Japanese — but recognizes they are under Japanese administration.

A rising economic and military power, China has recently become more assertive over its maritime claims.