A spat between President Donald Trump and Ukrainian President Volodymyr Zelenskyy has come as a windfall for the Kremlin, raising Moscow's hopes for a halt in the U.S. assistance to Kyiv even as Russian troops seek to extend their gains in Ukraine.

Trump took issue with Zelensky suggesting it would take time to come to an agreement to end the war. The Ukrainian leader also tried to offer a positive take on the U.S.-Ukraine relationship in the aftermath of last week's White House meeting.

President Volodymyr Zelenskiy said on Monday that Ukraine had to engage in "substantive" diplomacy to bring to a rapid end the three-year-old war with Russia, and he hoped to be able to rely on U.S. support in securing peace.