KIEV, Ukraine — Senior Ukrainian officials said they were blindsided over the summer when they heard the United States would withhold security assistance to the country.

“It was a total surprise,” said Pavlo A. Klimkin, who was Ukraine’s foreign minister in August when he learned of the Trump administration’s suspension of military aid by reading a news article.

The blocking of military aid to Ukraine is now at the center of questions about whether President Trump manipulated foreign policy to pressure the Ukrainian government to take action that would hurt Joseph R. Biden Jr., the former vice president and a top rival in the campaign for the presidency.

President Trump acknowledged on Sunday that he used a July 25 phone call with the new Ukrainian president, Volodymyr Zelensky, to accuse Mr. Biden of corruption.