By Patricia Zengerle

WASHINGTON (Reuters) – A senior U.S. senator demanded on Tuesday an investigation of the withholding of nearly $400 million in aid for Ukraine by President Donald Trump’s administration, as more congressional Democrats backed calls for impeachment action against Trump.

Senator Bob Menendez, the top Democrat on the Senate Foreign Relations Committee, said Congress had not been made aware of any substantive review of security assistance to Ukraine or any policy reason the funds should have been withheld.

In a letter to Mike Mulvaney, director of the White House Office of Management and Budget (OMB), Menendez said “it is becoming clear that” Trump put pressure on Ukrainian officials.

Menendez, in one of three letters sent to administration officials, also said “we must immediately understand whether, and to what extent, the President and his team converted duly-appropriated United States foreign assistance funds for his personal and political benefit, and what role federal agencies may have played in it.”

Arriving at the United Nations, Trump on Tuesday confirmed that he had wanted the money for Ukraine frozen, saying he wanted European countries to provide assistance to Kiev, but changed his mind after “people called me.”

However, Trump told reporters as he arrived at the United Nations that he still felt other nations should be paying to help Ukraine. “The money was paid, but very importantly, Germany, France, other countries should put up money,” Trump said.

Trump on Monday had denied trying to coerce Ukrainian President Volodymyr Zelenskiy in a July 25 phone call to launch a corruption investigation into political rival Joe Biden, the frontrunner for the 2020 Democratic presidential nomination, and Biden’s son in return for the U.S. military aid.

Trump on Tuesday indicated that he expects a “readout” of the phone call with Ukraine’s president to be made public.

“And when you see the call, when you see the readout of the call, which I assume you’ll see at some point, you’ll understand. That call was perfect. It couldn’t have been nicer,” Trump told reporters.

Separately on Tuesday, House of Representatives Speaker Nancy Pelosi, a Democrat, was to meet with senior Democrats to consider impeaching Trump. Under the U.S. Constitution, the House has the power to impeach a president for “high crimes and misdemeanors” and the Senate then holds a trial on whether to remove the president from office.

Pelosi had in the past opposed impeachment efforts but appeared to be moving closer as Democrats demand that the Trump administration release details of a whistleblower complaint and the transcript of his call with Ukraine’s president.

Trump on Tuesday accused Democrats of considering impeachment for purely political reasons.

“They have no idea how they stop me. The only way they can try is through impeachment,” Trump said at the United Nations.

In his letter, Menendez noted that the U.S. State and Defense Departments recommended and prepared to distribute in late June $391.5 million in military and security assistance to boost Ukraine’s armed forces as the country dealt with Russian aggression and sought to improve maritime security in the Black Sea.

However, weeks before Trump’s call with Zelenskiy, OMB blocked the aid, Menendez said in the letter to Mulvaney.

“Ukrainian officials were reportedly ‘blindsided,'” Menendez wrote. “For months, despite repeated inquiries from my office and others, administration officials have been unable to offer any policy justification for why these funds were blocked.”

OMB did not immediately respond to a request for comment.

In his letters, Menendez asked for information about who decided to withhold the funds, whether Trump directed that they be withheld, how the administration communicated between departments about the decision and what changed between the decision in June and the release of the money this month.

“I understand from State Department officials that the White House provided the Department with no official reason for delaying security assistance to Ukraine for almost three months, which is highly disconcerting,” Menendez wrote in a letter to Secretary of State Mike Pompeo.

A Senate aide told Reuters on Tuesday that officials from the State Department confirmed to Senate staff on Friday that neither the State Department nor the Pentagon had policy objections to the release of money to Ukraine. The aide said Senate staff had also been told that it was Mulvaney who directed the State department not to send the money to Ukraine.

(Reporting by Patricia Zengerle, additional reporting by Steve Holland at the United Nations; Editing by William Maclean and Will Dunham