Washington (AFP) - US President Donald Trump refused on Friday to rule out accepting Russia's annexation of Ukraine's Crimea region when he meets his Kremlin counterpart Vladimir Putin next month.

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.

Just days after Western leaders meet in Brussels on July 11, Trump is due to fly to Helsinki for his first one-on-one summit on July 16 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, unbadged Russian troops 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.

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.