The Group of Seven needs to take a "clear and tough stance" on China's controversial maritime claims and the Russian annexation of Crimea, European Council President Donald Tusk said Thursday.

Speaking on the sidelines of a G7 summit in Japan, Tusk warned that the credibility of the club of rich nations was on the line.

"The test of our credibility at the G7 is our ability to defend the common values that we share," he told reporters.

"This test will only pass if we take a clear and tough stance on every topic of our discussions here... I refer in particular to the issue of maritime security and the South and East China Seas and (the) Russia-Ukraine issue."

Tusk added: "If we are to defend our common values it is not enough these days to only believe in them. We also have to be ready to protect them."

Beijing has angered some of its Southeast Asian neighbours, including the Philippines and Vietnam, by claiming almost all of the South China Sea.

Beijing is also locked in a dispute with Japan over rocky outcroppings in the East China Sea, stoking broader concerns about China's growing regional might and threats to back up its claims with force, if necessary.

"The policy of the G7 is clear: any maritime or territorial claim should be based on international law and any territorial dispute should be resolved by peaceful means," Tusk said.

"Unilateral action and the use of force or coercion will not be accepted."

Russia and Ukraine have been locked in a bitter feud since Moscow annexed Crimea in 2014 and was then accused of fuelling a bloody separatist uprising in the east of the country.

The crisis has pushed ties between Russia and the West to their lowest point since the end of the Cold War, and drawn sanctions against Moscow.

"The European Union and the entire G7 continue to believe that this crisis can only be resolved in full compliance with... international law, especially the legal obligation to respect Ukraine's sovereignty, territorial integrity and independence," Tusk said.

Progress on the Minsk peace accords, designed to resolve the crisis, is slow, Tusk added.

"I want to state clearly that our stance vis-a-vis Russia, including economic sanctions, will remain unchanged as long as the Minsk agreements are not fully implemented," he said.

"Unfortunately there is much less progress on the implementation of Minsk than we had hoped for one year ago."