The White House broke the law by withholding aid approved for Ukraine by US Congress, an independent watchdog has found.
The report by the Government Accountability Office (GAO), a non-partisan federal agency, came as the impeachment trial of the US president formally opened in the Senate.
The freezing of the aid is at the centre of the case against Mr Trump.
The agency said that delaying Congress-approved aid for Ukraine for "policy reasons", rather than technical budgetary needs, violates the law.
"Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law," it said.
"The President is not vested with the power to ignore or amend any such duly enacted law."
Mr Trump has dismissed charges against him and has denied that he tried to influence officials.
Meanwhile, Ukraine has launched a criminal investigation to find out if former US ambassador Marie Yovanovitch came under illegal surveillance in her post last year.
The announcement from Ukraine's Interior Ministry said its position was "not to interfere in the domestic affairs" of the US, but that evidence suggested a possible violation of Ukrainian or international law.
It referred to a cache of documents released by Democrat politicians earlier in the week that are said to show Whatsapp records from Lev Parness, an associate of Mr Trump's lawyer Rudy Giuliani, discussing apparent physical and electronic monitoring of Ms Yovanovitch.
"The published records contain the fact of possible violation of the legislation of Ukraine and the Vienna Convention on Diplomatic Relations," the ministry statement said.
It added that such legislation was in place to protect "the rights of a diplomat in the territory of another country".
Ms Yovanovitch, who is at the centre of the ongoing US impeachment inquiry, was abruptly recalled from her position in Kiev in spring last year.
She was later described by Mr Trump in his now infamous phone call with Ukrainian President Volodymyr Zelenskiy as "bad news".
He also said: "Well, she's going to go through some things."
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Meanwhile on Thursday, the Ukrainian Interior Ministry said it had also requested help from the FBI to investigate what it believes was a cyber attack from Russian military hackers on energy company Burisma.
"It is noted that the hacker attack is probably committed by the Russian special services," the statement said, adding: "Persons involved in the commission of this criminal offence are identified."
The Ukrainian gas company has also been at the heart of the ongoing impeachment case after Mr Trump urged his counterpart in the phone call last year to investigate Hunter Biden, the son of his political rival Joe, over his time on the board of Burisma.
Trump was impeached last month on charges of abusing his power for pressuring Ukraine to investigate Democratic rivals, as he was withholding the aid, and for obstructing Congress' ensuing probe.
The president has dismissed the impeachment as a "con job".