Japanese prime minister Shinzo Abe has warned that Japan will respond with force if any attempt is made to land on disputed islands.
He was answering questions in parliament after eight Chinese surveillance ships entered Japanese territorial waters near the Senkaku Islands, which China calls the Diaoyu.
A flotilla of 10 fishing boats carrying Japanese activists was also reported to be in the area, as well as the Japanese coastguard.
The BBC reports the warning was the most explicit to China since Mr Abe took power in December.
Tokyo nationalised part of the island chain in September 2012. Taiwan also claims the islands.
The dispute has led to serious diplomatic tension between China and Japan.