BEIJING — Chinese state media said Thursday that four Japanese are being investigated after being accused of entering a military zone without authorization and illegally filming military targets.

The report comes as China and Japan are in sharp diplomatic dispute over the detention of a Chinese fishing boat captain near islands both countries claim as their own.

China’s state-run Xinhua News Agency cited state security authorities in the northern city of Shijiazhuang as saying they have "taken measures" against the four Japanese "after receiving a report about their illegal activities." There was no elaboration.

The authorities accuse the Japanese of entering a military zone without authorization in Hebei province, the capital of which is Shijiazhuang.

The brief report late Thursday night did not say whether the four Japanese are in detention.

The news could further raise tensions between the two Asian powers. China-Japan relations are at their worst in several years after Japan arrested the Chinese captain whose fishing boat collided with Japanese coast guard vessels two weeks ago near the disputed islands in the East China Sea.

Relations slipped further after Japan extended the detention of the Chinese captain Sunday. China quickly suspended high-level contacts with Japan and announced that Premier Wen Jiabao would not be meeting with Japanese Prime Minister Naoto Kan during U.N. meetings in New York this week.

On Tuesday, Wen threatened "further action" against Japan if it did not release the Chinese captain immediately.

Tokyo "bears full responsibility for the situation, and it will bear all consequences," Wen told a gathering of overseas Chinese this week, according to China’s Foreign Ministry website.

The report did not elaborate on what actions China might take.

The United States on Thursday urged Japan and China to quickly resolve the dispute.

Secretary of State Hillary Rodham Clinton told new Japanese Foreign Minister Seiji Maehara that good ties between China and Japan are crucial to Asia’s prosperity, State Department spokesman P.J. Crowley said.

"Neither side wants to see the situation escalate to the point where it has long-term regional impact," Crowley said in New York.

The dispute faces a test on Sept. 29, the deadline by which Japanese prosecutors must decide whether to charge the Chinese captain. Fourteen crew members and the boat have been returned.