Laura Cooper, the Pentagon official in charge of the country, testified to Congress that Ukraine reached out on July 25, the very day that Trump spoke to his counterpart Volodymyr Zelensky in a call that triggered an impeachment inquiry.

Under oath before the House Intelligence Committee, Cooper said she received emails on July 25 saying that both the Ukrainian embassy in Washington and the House Foreign Affairs Committee were asking about the assistance.

"I would say that, specifically, the Ukrainian embassy staff asked, 'What is going on with Ukrainian security assistance?" Cooper said.

Asked by Representative Adam Schiff if the Ukrainians had been "concerned," Cooper replied, "Yes, sir."

Trump is accused of improperly withholding aid and a summit with Zelensky as he pressed Ukraine to launch an investigation into his domestic rival Joe Biden.

Trump in the phone call asked Zelensky for "a favor." Trump's defenders have argued, in part, that his effort was not improper as Ukraine did not know about the holdup.

Kurt Volker, the US pointman on Ukraine, has said that he was not aware of the aid freeze until Politico ran an article in late August.

Cooper said that her staff received the emails on July 25 and that she had not been personally briefed on them until she conducted research in preparation for her congressional appearance.