US President Donald Trump was under renewed pressure over his dealings with Ukraine on Sunday after it emerged that another whistleblower had come forward, this time with firsthand knowledge of the allegations.

News of the second whistleblower broke amid signs of increasing anxiety among Republican senators, as the Democrats in the House of Representatives pressed ahead with impeachment proceedings.

The existence of the second whistleblower was announced by the legal team representing the original complainant who accused Mr Trump of pressuring Ukraine's president to investigate his Democratic rival Joe Biden.

“I can confirm that my firm and my team represent multiple whistleblowers in connection to the underlying August 12, 2019, disclosure to the Intelligence Community Inspector General,” Andrew Bakaj, managing partner of Compass Rose Legal Group, wrote on Twitter.

Mark Zaid, another member of the original whistleblower’s legal team, said the second individual had spoken to the inspector general of the intelligence community but had not filed a formal complaint.