Two state-owned Chinese ships entered into territorial waters around disputed islands in the East China Sea on Monday, Japan's coastguard said.
One of the ships entered into waters surrounding the islands known as the Senkakus in Japan and Diaoyus in China at around 1:32pm (0432 GMT), while the other followed about 20 minutes later, the coastguard said.
The ships were spotted by coastguard aircraft which were patrolling the region, it said, without elaborating further.
A coastguard spokesman could not be reached for immediate comment.
China has repeatedly sailed into waters around the disputed islands since Japan nationalised the chain in September.
Beijing sent its ships into the area as recently as December 21, after the conservative Liberal Democratic Party swept to a landslide election victory in Japan.
Newly elected Prime Minister Shinzo Abe, who officially assumed the post on Wednesday, has sought to mend Tokyo's ties with Beijing.
But analysts have long predicted that China is unlikely to change its claim to the islands and will probably continue to send its ships into the surrounding waters to bring the world's attention to the dispute.
Earlier in December, a state-owned Chinese plane breached Japanese airspace over the islands. Tokyo responded by scrambling fighter jets and said it was the first time Beijing had breached its airspace since 1958.