Moscow (AFP) - The Kremlin on Monday said Vladimir Putin and Donald Trump can discuss "all other issues" except Crimea, the peninsula Moscow annexed from Ukraine in 2014, at their highly anticipated first summit this month.

Putin has "repeatedly stated and explained that Crimea cannot be and will never be on the agenda because it is an inseparable part of Russia," Kremlin spokesman Dmitry Peskov told reporters.

"All other issues are ones of consensus, discussion and finding possible points of contact," he added.

Peskov also said that while Moscow and Washington "differ" on "many" issues, "political will has prevailed and understanding is growing."

On Saturday, Trump refused to rule out accepting Russia's annexation of Crimea when he meets Putin for a first one-on-one summit of the two leaders in Helsinki on July 16.

Asked by reporters whether he was considering -- as has been reported -- dropping Washington's opposition to the 2014 land grab, Trump said: "We're going to have to see."

Trump's refusal to reaffirm the long-standing US opposition to Russia's intervention in Ukraine will dismay European allies ahead of next month's NATO summit on July 11, just days before his meeting with Putin.

Accepting that Crimea is never to return to Ukrainian control would be a major concession to Russia, which is languishing under tough international trade sanctions over its actions.

In early 2014, with areas of Ukraine falling into chaos in the wake of a Kiev street revolt that ousted a pro-Russian president, Russian troops -- in unmarked uniforms -- seized Crimea.

A referendum was called in the territory, which has a large Russian-speaking population, and on March 18, 2014 Russia formally annexed it to the Russian federation.

Ukraine, which is also facing a pro-Russian rebellion on its mainland in the Donbas region, furiously opposed this breach of its sovereignty -- with stern Western backing.

Trump has reluctantly approved the dispatch of US anti-tank missiles to support Ukraine forces, and other senior US officials continue to insist sanctions will remain until Moscow backs down.

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But Trump has long supported warmer relations with Russia, and Western diplomats based in Washington have begun to fear that he could make major concessions in Helsinki.