TOKYO (AFP) - Chinese ships sailed into disputed waters off Japan-administered islands in the East China Sea on Friday, officials said, after G7 leaders cautioned against "intimidation" in territorial disputes.

The Japanese coastguard said two vessels entered the 12-nautical-mile band of territorial waters around one of the Senkaku islands, which China also claims and calls the Diaoyus, at 10am (0100 GMT).

"The two ships were initially spotted about 27km west of Uotsurijima, but they have now entered the Japanese territorial waters," a coast guard official said.

For over a year after Tokyo nationalised some of the islands in September 2012, Chinese vessels and aircraft regularly approached, playing cat and mouse with the Japanese coastguard.

There has been a marked decrease in incidents this year - Friday's was only the 13th - although each episode has tended to come in apparent response to events in the larger relationship.

The Chinese move came after leaders of the Group of Seven, who met in Brussels, warned against any use of force in the East and South China Sea, where Beijing is involved in disputes with several countries.

"We are deeply concerned by tensions in the East and South China Sea," the leaders said in a statement at the end of a first day of talks in the Belgian capital.

"We oppose any unilateral attempt by any party to assert its territorial or maritime claims through the use of intimidation, coercion or force."

The statement, which did not mention any country by name, called on all countries to follow international law in resolving competing claims.

In recent months, China has ramped up its activity in the South China Sea, which Beijing claims almost in its entirety.

Chinese and Vietnamese vessels have sparred near the site of a drilling rig that Beijing moved into disputed waters, with one boat sinking after an apparent collision.