Former vice-president says Congress must hold the president accountable.
Former vice-president and presidential candidate Joe Biden has broken his silence about allegations that President Donald Trump wanted an investigation into his son's business dealings in Ukraine.
He was speaking just hours after the White House released an official summary of the call between Trump and Zelenskiy, in which Trump asks the Ukrainian leader to look into Biden and his son, Hunter, who worked for a company that was drilling for oil in Ukraine.
"There's a lot of talk about Biden's son, that Biden stopped the prosecution and a lot of people want to find out about that, so whatever you can do with the attorney general would be great," Trump said in the call, according to the summary provided by the justice department.
"Biden went around bragging that he stopped the prosecution so if you can look into it... it sounds horrible to me," Trump said, according to the memo.
The call prompted the U.S. House of Representatives to announce the beginning of impeachment proceedings.
In the statement, Biden said: "It is a tragedy for this country that our president put personal politics above his sacred oath.
"He has put his own political interests over our national security interest, which is bolstering Ukraine against Russian pressure.
"It is an affront to every single American and the founding values of our country. This is not a Republican issue or a Democratic issue. It is a national security issue. It is a test of our democratic values.
"Congress must pursue the facts and quickly take prompt action to hold Donald Trump accountable.
"In the meantime, I will continue to focus my campaign not on how Donald Trump abused his power to come after my family, but on how he has turned his back on America's families."
It came after Trump had ordered the US government to freeze nearly $400 million in American aid to Ukraine.
House of Representatives Speaker Nancy Pelosi on Tuesday announced that the Democratic-led House was moving forward with an official impeachment inquiry and directed six committees to proceed with investigations of the president's actions.
Democrats have accused Trump, who is seeking re-election next year, of soliciting Ukraine's help to smear Biden, the front-runner for the Democratic presidential nomination, before the 2020 election.
The current controversy arose after a whistleblower from within the US intelligence community brought a complaint with an internal watchdog relating to Trump's conversation with Zelenskiy.
Even though federal law calls for such complaints to be disclosed to Congress, the Trump administration has refused to do so.