Ukrainian President Volodymyr Zelenskiy said Thursday there was "no blackmail" in his July phone call with U.S. President Donald Trump that is now at the center of an impeachment inquiry by the Democrat-led U.S. House of Representatives.

Zelenskiy told reporters he was unaware at the time of the call that Trump had moved to withhold hundreds of millions of dollars in military aid to Ukraine.

Trump says he made that decision to push Ukraine to tackle corruption. Democrats accuse the U.S. leader of freezing the aid to pressure Zelenskiy's administration into investigating Democratic presidential candidate Joe Biden and his son, Hunter.

Zelenskiy said Thursday his goal in the call was to arrange a future meeting with Trump and to get him to change his public rhetoric about Ukraine being a corrupt country. The Ukrainian leader said the U.S. side did not set any conditions for a meeting.

According to a whistleblower complaint, and a transcript of the call released by the White House, Trump urged Zelenskiy to open an investigation into Biden.

The three main House committees conducting the impeachment inquiry released a set of text messages obtained from former U.S. envoy to Ukraine Kurt Volker, including one Volker sent to Zelenskiy aide Andrey Yermak saying, "We will nail down date for visit to Washington" if Zelenskiy "convinces Trump he will investigate."

Volker gave closed-door testimony to the House Intelligence, Foreign Affairs and Oversight committees last week. The State Department blocked U.S. Ambassador to the European Union Gordon Sondland from giving planned voluntary testimony on Tuesday, and it is uncertain if the committees will be able to go ahead with their scheduled Friday meeting with former Ambassador to Ukraine Marie Yovanovitch.

Trump has denied wrongdoing and described his conversation with Zelenskiy as a "perfect call."

"This is a con job. This is a con being perpetrated on the United States public and even the world," Trump told reporters Wednesday.

The president spoke a day after the White House said in a letter it is refusing to participate in the Democratic-led inquiry into whether he should be impeached.

Trump did not say exactly what House leaders must do if they want his cooperation. But he said he would cooperate "if they give us our rights" and Republicans "get a fair shake."

Among the gripes spelled out in the White House letter is a complaint that Democrats are denying Trump and his Republican supporters in the House the opportunity to question witnesses and see the evidence the Democrats have.

The White House calls the impeachment inquiry "unconstitutional" and demands the full House be allowed to vote on whether there should be an inquiry.

But there is no rule preventing the House from looking into allegations of illegal activity by a president before deciding whether to bring actual articles of impeachment to a vote.

Speaker of the House Nancy Pelosi called the White House letter "the latest attempt to cover up his [Trump's] betrayal of our democracy and to insist that the president is above the law."

She says Democrats would consider a refusal to cooperate more evidence of obstruction.

Trump has accused Biden of corruption, alleging that when he was vice president, he threatened to withhold loan guarantees to Ukraine unless the government stops investigating a gas company for which Biden's son, Hunter, held a seat.

There is no evidence of any wrongdoing by Joe or Hunter Biden.