President Donald Trump on Tuesday confirmed new reporting that he moved to hold back military aid from Ukraine a week ahead of a call with Ukrainian President Volodymyr Zelensky.

The substance of that call is now at the center of an explosive whistleblower complaint against Trump.

Trump has acknowledged that he discussed investigating former Vice President Joe Biden and his son, Hunter, during the call with Zelensky.

But he rejected the notion he dangled military aid over Ukraine to urge it toward investigating Biden, who is the 2020 Democratic frontrunner.

Trump said on Tuesday that his main concern with sending military aid to Ukraine is that other countries aren't paying their fair share.

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President Donald Trump confirmed on Tuesday that he decided to withhold military aid from Ukraine days before a phone call with its recently elected president, Volodymyr Zelensky.

That call is now at the center at an escalating scandal linked to an unprecedented whistleblower complaint against Trump from a member of the US intelligence community.

Politico first reported the US's move to withhold the aid. And on Monday evening, the Washington Post reported on the timing of Trump's decision, which came roughly a week before his July 25 phone call with Zelensky.

The Post reported that Trump told his acting chief of staff, Mick Mulvaney, to put the brakes on approximately $400 million in military aid for Ukraine before the call.

And on Sunday, the president confirmed he discussed investigating former Vice President Joe Biden and his son, Hunter, during the conversation with Zelensky. But he rejected the notion he dangled military aid over Ukraine to urge it toward investigating Biden, a top political opponent and the 2020 Democratic frontrunner.

Trump said on Tuesday that his main concern with providing aid to Ukraine is that other countries are not paying their fair share.

"I want other countries to put up money. I think it's unfair we put up the money. Then people called me and said let it go. I let it go," Trump said. "We paid the money. The money was paid. But very importantly Germany, France, other countries should put up money and that's been my complaint from the beginning."

The Wall Street Journal reported last week that during the phone call, Trump pressed Zelensky at least eight times to work with his personal lawyer, Rudy Giuliani, to investigate the Bidens over their dealings in Ukraine.

The younger Biden sat on the board of Burisma Holdings, a Ukrainian gas-extraction company, from 2014 to early this year. Trump and Giuliani accused the elder Biden of trying to stymie a criminal investigation into Burisma in 2016 by pushing the Ukrainian government to fire Viktor Shokin, the prosecutor general leading the inquiry.

While Biden did seek the prosecutor's removal, those accusations are unsubstantiated, with government officials and Ukrainian anticorruption advocates in fact saying that Shokin had hampered the investigation into Burisma long before Biden had entered the picture, according to The Journal.

Trump denied suggestions that he told the Ukrainians that the military funds were conditioned upon investigating Biden. But he appeared to acknowledge on Monday that the aid package was at least somewhat on his mind during the call with Zelensky.

"It's very important to talk about corruption," Trump said. "If you don't talk about corruption, why would you give money to a country that you think is corrupt? It's very important that on occasion, you speak to somebody about corruption."