A second whistleblower has come forward in the impeachment case against US president Donald Trump, according to a lawyer representing the original whistleblower.
Mark Zaid - who filed a formal complaint with the inspector general last month, triggering the impeachment inquiry - told The Associated Press that the second whistleblower, who also works in intelligence, had not filed a complaint with the inspector general but had "firsthand knowledge that supported" the original whistleblower.
The original whistleblower complained that Trump was "using the power of his office to solicit interference from a foreign country" in the 2020 US election.
Trump and his supporters have rejected the accusations that he did anything improper.
Earlier, US Secretary of State Mike Pompeo said his department intended to follow the law in the House impeachment investigation and vigorously defended Donald Trump, dismissing questions about the president's attempts to push Ukraine and China to investigate a Democratic political rival.
The Trump administration and House Democrats often disagree about what the law requires, leaving open the question of how Mr Pompeo may interpret Democrats' demands for key information about Mr Trump's handling of Ukraine.
Mr Pompeo, speaking on Saturday in Greece, said the State Department sent a letter to Congress on Friday night as its initial response to the document request, and added: "We'll obviously do all the things that we're required to do by law."
He is allowing Democrats to interview a series of witnesses next week, among them Gordon Sondland, the US ambassador to the European Union, who is another key figure in the probe.
The administration has struggled to come up with a unified response to the quickly progressing investigation.
Democrats have warned that defying their demands will in itself be considered "evidence of obstruction" and a potentially impeachable offence.
Mr Pompeo has become a key figure in the Democrats' investigation.
He was on the line during the July phone call in which Mr Trump pressed Ukrainian President Volodymyr Zelenskiy to investigate former vice president Joe Biden and his son Hunter - sparking a whistleblower complaint and now the impeachment inquiry.
Mr Pompeo had initially tried to delay a handful of current and former officials' co-operation with the inquiry and accused Democrats of trying to "bully" his staff.
On Saturday, Mr Pompeo did not back off in his defence of Mr Trump's call with Ukraine.
"There has been some suggestion somehow that it would be inappropriate for the United States government to engage in that activity and I see it just precisely the opposite," he said.
Mr Trump has offered a series of contradictory statements when it comes to the Democrats' subpoena of White House records.
Asked on Wednesday whether the White House intended to comply, Mr Trump told reporters "I always cooperate", even as he dismissed the inquiry as "a hoax".
But a day later he had a different answer for the same question, saying he would instead leave the matter to his lawyers.
"That's up to them to decide," he said.
"But the whole investigation is crumbling."
By Friday, however, the president confirmed reports that the White House was preparing a letter to House Speaker Nancy Pelosi, arguing that Congress cannot undertake an impeachment investigation without first having a vote to authorise it.
Ms Pelosi has insisted the House is well within its rules to conduct oversight of the executive branch under the Constitution regardless.
It was unclear on Saturday when or if that letter would be sent.
