Nancy Pelosi's announcement came after row over Trump's phone call with Ukraine President

The U.S. House of Representatives will launch an inquiry to investigate whether to impeach U.S. President Donald Trump for seeking foreign help to smear a political rival, a senior Democratic lawmaker said on Tuesday.

Richard Neal, chairman of the House Ways and Means Committee, made the comment to reporters after meeting with House Speaker Nancy Pelosi. Neal did not offer details on how the formal inquiry involving the possible impeachment of the Republican president would be pursued.

Meanwhile, Trump promised to release a transcript of his phone call with Ukraine's president at the centre of the controversy.

Pelosi, who for months had been reluctant to embrace an impeachment effort to try to remove Trump from office, is scheduled to make a public statement at 5 p.m. ET (2100 GMT). Pelosi's fellow Democrats control the House.

Trump on Tuesday confirmed he had withheld nearly $400 million in U.S. aid to Ukraine but denied he did so as leverage to get Ukrainian President Volodymyr Zelenskiy to initiate an investigation that would damage Joe Biden, the front-runner for the 2020 Democratic presidential nomination.

During an appearance at an event sponsored by The Atlantic magazine, Pelosi, the top Democrat in Congress, declined to say what action she planned, but told the audience, "It's absolutely, absolutely essential that the president be held accountable. No one is above the law."

Biden on Tuesday called on Trump to fully comply with congressional investigations into the matter or risk impeachment.

"If he continues to obstruct Congress and flout the law, Donald Trump will leave Congress in my view with no choice but to initiate impeachment proceedings," Biden told reporters in Wilmington, in his home state of Delaware.

Trump, who has withstood repeated scandals since taking office in January 2017, said a "complete, fully declassified and unredacted" transcript of the July 25 call would be released on Wednesday.

The controversy came to light after a whistleblower from within the U.S. intelligence community lodged a complaint with an internal watchdog about Trump's conversation with Zelenskiy.

House Intelligence Committee Chairman Adam Schiff said his panel is communicating with an attorney representing the whistleblower and that individual would like to testify this week.

Trump said the transcript would show the call was "totally appropriate," that he had not pressured Zelenskiy to investigate Biden and that there had been no "quid pro quo" for U.S. aid in exchange for a probe. Quid pro quo is a Latin phrase meaning a favour that is exchanged for a favour.

But Pelosi, who met during the day with fellow Democratic lawmakers to consider impeachment, said it would be improper for Trump to ask a foreign leader to investigate a political opponent like Zelenskiy even if there is not an explicit quid pro quo.

"If the president brings up, he wants them to investigate something on his political opponent, that is self evident that it is not right. You don't ask foreign governments to help us in our elections. That's what we tried to stop with Russia," Pelosi said.

Trump has produced no evidence of wrongdoing by Biden or his son.

In remarks to reporters over the intensifying controversy, Trump indicated there was nothing sinister about withholding the aid but that he wanted Europe and not just the United States to step up and provide Ukraine assistance.

The money was later released by the Trump administration.

Trump is seeking re-election in November 2020.

Trump on Sunday acknowledged that he had discussed Biden and Biden's son Hunter, who had worked for a company drilling for gas in Ukraine, with Zelenskiy.

But Trump on Monday denied trying to coerce Zelenskiy in the phone call to launch a corruption investigation into Biden and his son in return for the U.S. military aid.

Arriving at the United Nations before his speech to the annual General Assembly on Tuesday, Trump confirmed that he had wanted the money for Ukraine frozen, but changed his mind after "people called me."

Trump told reporters that he 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 added, "We're putting up the bulk of the money, and I'm asking why is that? ... What I want, and I insist on it, is that Europe has to put up money for Ukraine also."

The United States has been providing military aid to Ukraine since Russia annexed Crimea in 2014. The $391.5 million in aid at issue in the current controversy was approved by the U.S. Congress to assist Ukraine in dealing with an insurgency by Russian-backed separatists in the eastern part of the country.

Trump's explanation for withholding the aid differed from one he offered a day earlier when he said, referring to Ukraine, "We want to make sure that country is honest," and "Why would you give money to a country that you think is corrupt?"

About the call itself, Trump told reporters on Tuesday: "...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."

The House will vote Wednesday on a resolution of disapproval of the Trump administration's effort to block the release of the whistleblower complaint, Pelosi and House Majority Leader Steny Hoyer announced.

U.S. intelligence agencies and a special counsel previously concluded that Russia boosted Trump's 2016 presidential election bid with a campaign of hacking and propaganda aimed at harming his Democratic opponent Hillary Clinton.

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.

IMPEACHMENT EFFORTS

Under the U.S. Constitution, the House has the power to impeach a president for "high crimes and misdemeanours" and the Senate then holds a trial on whether to remove the president from office. No president has ever been removed from office through impeachment. Democrats currently control the House and Republicans control the Senate.

A House committee has already launched a formal impeachment probe of Trump in light of his actions in the Russia matter but the impeachment drive never won the support of key party figures including Pelosi.

Trump accused Democrats of considering impeachment for purely political reasons.

"I'm leading in the polls. They have no idea how they stop me. The only way they can try is through impeachment," Trump said at the United Nations, although opinion polls show Biden actually leading Trump.

Three House committees wrote to White House Counsel Pat Cipollone saying they would issue a subpoena if documents related to Trump's dealings with Ukraine are not turned over by Thursday.

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 wrote.

Joseph Maguire, acting U.S. director of national intelligence, is defying a federal law mandating that the whistleblower report be shared with Congress. Maguire is due to testify at a public House intelligence committee hearing on Thursday.

The top Senate Democrat, Chuck Schumer, said Trump's plan to release the transcript was not enough and that lawmakers need to see the whistleblower complaint itself.