China will have no option but to "react strongly" if Japan intensifies its provocative actions over China's Diaoyu Islands, a Foreign Ministry official has said.

"Japan should be clear about that," the official from the ministry's Asian Affairs Department said, lodging a protest with Japan's embassy in Beijing on Tuesday.

He was reacting to Japanese media reports that Japan's Coast Guard had anchored patrol vessels carrying helicopters in the waters of Diaoyu Islands since Feb 1, the ministry said on its website yesterday. "If the reports are true, it would constitute a severe violation of Chinese territorial sovereignty," the official said. "We express strong concern over that."