But Trump is known for his spontaneous and sometimes erratic style of doing diplomacy, and Kyiv fears that behind closed doors he may prove susceptible to Putin's efforts to win U.S. acceptance of Russia's 2014 annexation of Crimea from Ukraine, Reuters reported.

Trump and Putin will discuss a range of issues including arms control, Syria and U.S. economic sanctions against Moscow at their summit, which could potentially prove a game-changer for Ukraine.

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"We received clear assurances from the American side that Ukrainian interests will be confirmed and protected (at the summit)," said a Ukrainian diplomat, speaking on condition of anonymity because of the sensitivity of the matter.

"It is important for us that no nasty surprise happens behind our backs, and we received assurances (from the U.S. side) that this will not happen."

Kyiv has counted on U.S. diplomatic and financial support in its standoff with Moscow since the annexation of Crimea and the outbreak of Russian-backed separatist fighting in eastern Ukraine that has killed more than 10,000 people despite a notional ceasefire.

Kyiv and Washington accuse Russia of sending troops and arms to Ukraine's eastern Donbas region, which Moscow denies.

Ukraine hopes Trump will push Putin to scale back Moscow's support for the separatists in eastern Ukraine and to soften his position regarding the deployment of a peacekeeping mission to that region.