Washington (CNN) President Donald Trump downplayed on Friday a complaint submitted by an intelligence whistleblower that reportedly involves Ukraine, claiming the complainant is partisan and his conversations with foreign leaders are "appropriate."

But in a back-and-forth with reporters, he did not deny discussing former Vice President Joe Biden with Ukrainian President Volodymyr Zelensky in a July phone call -- a potential subject for the complaint.

And he admitted he did not know the whistleblower's identity, despite his accusation of partisanship.

It was a robust self-defense against accusations that remain, for now, veiled in mystery. An intelligence watchdog has said the claims are "urgent" and "serious," but the acting top intelligence official has declined to provide them to Congress.

Trump dismissed them as another attempt to bring down his presidency. Speaking in the Oval Office, Trump called the story "ridiculous" and described the whistleblower as "just another political hack job."

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