Leaders of the Group of Seven industrialised nations said Thursday they were deeply concerned by tensions in the East and South China Sea.

The G7 leaders meeting in Brussels warned against any use of force in the area, where the United States has warned Beijing over increasing territorial assertiveness.

"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.

Japan nationalised the Senkaku islands in the East China Sea, which China also claims and calls the Diaoyus, in 2012, setting off a tense confrontation with Beijing.

China is also locked in territorial disputes, in the South China Sea, which Beijing claims almost entirely.

Brunei, Malaysia, the Philippines and Vietnam claim parts of the sea, with Manila and Hanoi being the most vocal in opposing China's claims.

Taiwan is also a claimant.

US Defence Secretary Chuck Hagel in May accused China of "destabilising actions" in the South China Sea.