BERLIN — President Obama and several European leaders “unanimously agreed” on Friday to keep sanctions in place against Russia for its intervention in Ukraine, amid concern that President-elect Donald J. Trump would soften the United States’ stance against Moscow.

The show of solidarity came as American allies — and Ukrainians themselves — have been unsettled by uncertainty regarding what kind of foreign policy Mr. Trump will pursue.

With surging populist movements straining alliances and Mr. Trump’s election upending the political calculations of many countries, Ukraine may be among the most vulnerable to the shifting political winds.

Fighting in Ukraine has continued since Moscow stealthily fomented an uprising among ethnic Russians in eastern Ukraine with the help of undercover Russian forces, and then annexed Crimea in March 2014.