'I said nothing wrong': Trump defends call amid claim he asked Ukraine leader to investigate rival Joe Biden's son

US President Donald Trump urged the new leader of Ukraine this summer to investigate the son of former Vice President Joe Biden, a person familiar with the matter said.

Democrats condemned what they saw as a clear effort to damage a political rival, now at the center of an explosive whistleblower complaint against Mr Trump.

It was the latest revelation in an escalating controversy that has created a showdown between congressional Democrats and the Trump administration, which has refused to turn over the formal complaint by a national security official or even describe its contents.

Mr Trump is defending himself against the intelligence official's complaint, asserting that it comes from a "partisan whistleblower," though the president also says he doesn't know who had made it.

The complaint was based on a series of events, one of which was a July 25 call between Mr Trump and Ukrainian President Volodymyr Zelenskiy, according to a two people familiar with the matter. The people were not authorised to discuss the issue by name and were granted anonymity.

In a tweet today, Mr Trump referred to "a perfectly fine and routine conversation I had" with Ukraine's leader.

"Nothing was said that was in any way wrong."

According to one of the people, who was briefed on the call, Mr Trump urged Mr Zelenskiy to probe the activities of potential Democratic rival Mr Biden's son Hunter, who worked for a Ukrainian gas company.

Mr Trump did not raise the issue of US aid to Ukraine, indicating there was not an explicit quid pro quo, according to the person.

In an interview with Ukrainian outlet Hromadske, the foreign minister said his country was not interested in taking sides in US politics, but that Mr Zelenskiy had the right to keep secret the contents of his conversation with Mr Trump.

READ MORE: Joe Biden haunted by his past positions in Democratic debate

"I know what the conversation was about and I do not think there was any pressure" from Mr Trump, Vadym Prystaiko was quoted as saying.

"There was a conversation, different conversation, leaders have the right to discuss any existing issues.

"This was a long and friendly conversation that touched on a lot of issues, sometimes requiring serious answers."

Mr Biden said if the reports are true, "then there is truly no bottom to President Trump's willingness to abuse his power and abase our country".

He said Mr Trump should release the telephone transcript "so that the American people can judge for themselves".

The US government's intelligence inspector general has described the whistleblower's August 12 complaint as "serious" and "urgent".

Mr Trump insisted "it's nothing" and "just another political hack job".

The president said he has conversations with many leaders.

"It's always appropriate. Always appropriate," Mr Trump said.

"At the highest level always appropriate. And anything I do, I fight for this country."

Mr Trump was asked whether he knew if the whistleblower's complaint centred on the July 25 call with Mr Zelenskiy.

"I really don't know," Mr Trump said.

READ MORE: Joe Biden 'only Democratic candidate with wide lead over Trump': poll

When questioned whether he had brought up Mr Biden in the call, Mr Trump said, "It doesn't matter what I discussed".

But then Mr Trump urged the media "to look into" Mr Biden's background with Ukraine.

There has yet to be any evidence of any wrongdoing by Mr Biden or his son regarding Ukraine.