The former US special envoy for Ukraine told House investigators on Thursday that he warned Donald Trump's personal attorney, Rudy Giuliani, that Mr Giuliani was receiving untrustworthy information from Ukrainian political figures about former vice president Joe Biden and his son, according to two people familiar with his testimony.

Kurt Volker, who resigned last week after being named in a whistleblower complaint that sparked the House impeachment inquiry of Mr Trump, said he tried to caution Mr Guiliani that his sources, including Ukraine's former top prosecutor, were unreliable and that he should be careful about putting faith in the prosecutor's theories, said the people, who spoke on the condition of anonymity to discuss the closed-door meeting.

Mr Volker's testimony offers the first inside account of the Trump administration's efforts to press for a Ukrainian investigation into Mr Biden, who as a leading 2020 Democratic candidate to challenge Mr Trump for the White House has become a fixed target of the president's attacks.

Democrats came away from the day-long deposition convinced that documents Mr Volker provided to House investigators provide "ample evidence," representative Eric Swalwell said, that the Trump administration planned to require Ukraine's president to investigate the Biden family's ties to the Ukrainian energy giant Burisma and look into the 2016 election to "exonerate Russia's role," if the foreign leader wanted to meet with the American president.

At the heart of Mr Trump's effort is Mr Giuliani's contention that, as vice president, Mr Biden pushed for the firing of Ukraine's then-prosecutor general, Viktor Shokin, as part of a corrupt plot to halt investigations of a Ukrainian natural-gas company that employed Mr Biden's son Hunter.

Everyone Trump has fired or forced out Show all 13 1 /13 Everyone Trump has fired or forced out Everyone Trump has fired or forced out John Bolton Trump claimed to have fired Bolton, his national security adviser, while Bolton claimed he offered to resign. An anonymous White House source that Bolton's departure came as a result of the national security adviser working too independently of the president AFP/Getty Everyone Trump has fired or forced out Anthony Scaramucci Scaramucci lasted only six days in his role as Trump's communications director before being fired by John Kelly, the incoming chief of staff Getty Everyone Trump has fired or forced out Rick Perry Rick Perry announced his resignation just as he became embroiled in the president's impeachment scandal. The White House said Mr Perry was asked by Donald Trump to work with Rudy GIuliani in regards to Ukraine. AP Everyone Trump has fired or forced out Rex Tillerson Tillerson, Trump's first secretary of state, was fired after a series of clashes with the president over policy Getty Everyone Trump has fired or forced out James Mattis Mattis served as secretary of defense from the beginning of Trump's administration until retiring on 1 January 2019. However, the president later claimed that he had "essentially fired" Mattis Getty Everyone Trump has fired or forced out James Comey Comey was fired as director of the FBI early in Trump's presidency after serving in the role for four years prior. His dismissal is widely thought to have been related to the Russia investigation Getty Everyone Trump has fired or forced out Reince Priebus Priebus, Trump's first chief of staff, was forced out after six tumultuous months AFP/Getty Everyone Trump has fired or forced out David Shulkin Veterans affairs secretary Shulkin claims that he was fired, the White House claims that he resigned Getty Everyone Trump has fired or forced out John Kelly Kelly, Trump's second chief of staff, was forced out after 17 months in office. His departure was a confused affair though it is clear that Trump wanted Kelly out AFP/Getty Everyone Trump has fired or forced out Michael Flynn Flynn lasted 24 days as Trump's national security adviser before being fired for lying to the FBI Getty Everyone Trump has fired or forced out Lee Cisna Cisna served as director of citizen and immigration services between October 2017 and June 2019 before being asked to resign amid a major personnel change in the department of homeland security Everyone Trump has fired or forced out Madeline Westerhout Westerhout served as Trump's personal assistant after leaking private information about his family AFP/Getty Everyone Trump has fired or forced out Mira Ricardel Ricardel was forced out of her role as Deputy National Security Advisor after first lady Melania Trump publicly called for her to be fired

Joe Biden and his defenders have denied the accusation and noted that Mr Biden's push to remove Mr Shokin was part of a broader international effort that included the European Union and the International Monetary Fund, where leaders viewed Mr Shokin as inept.

In an interview on Thursday night, Mr Giuliani said that he was aware of "conversations" between US and Ukrainian officials about a statement that would commit the Ukrainians to "investigate everything, including Burisma." But Mr Giuliani stressed that "to my knowledge, it was never put out."

The New York Times reported earlier that Mr Volker and US ambassador to the European Union Gordon Sondland drafted the statement for Ukraine's president.

CNN later reported that the draft originated with the Ukrainians and was shared with Mr Volker, Mr Sondland and Mr Giuliani.

The State Department did not immediately respond to a request for comment. Mr Volker did not respond to multiple requests for comment.

Mr Volker also told lawmakers on Thursday that he and other State Department officials cautioned the Ukrainians to steer clear of US politics. Getting involved, he said he told them, would open the nation to allegations that it was interfering in an American election and could be detrimental to Ukraine long-term, according to these two individuals.

Mr Volker faced hours of questioning on Thursday from members of the House committees leading an impeachment inquiry into Mr Trump, the first of five former and current State Department officials to testify as part of the probe.

In discussing Mr Giuliani's sources, Mr Volker specifically mentioned former Ukrainian prosecutor general Yuri Lutsenko, a controversial figure in Ukrainian politics due in part to his battles with the country's anti-corruption bureau.

Andrew Weiss, the vice president for studies at the Carnegie Endowment for International Peace, said trusting Mr Lutsenko was a risky move given his status as a "discredited Ukrainian law enforcement figure."

"This is Ukraine politics 101," Mr Weiss said.

Mr Volker was named in the whistleblower complaint as the diplomat who set up a meeting between Mr Giuliani and a top adviser to Volodymyr Zelensky, the Ukrainian president, amid Mr Trump's effort to compel an investigation of the Bidens.

House investigators asked Mr Volker if that pressure included withholding a leader-level meeting with Mr Zelensky and about $400m (£324m) in military aid from the country, those familiar with the meeting said.

Mr Volker acknowledged, these people said, that the Trump administration had extended an invitation to Mr Zelensky shortly after his election in the spring and that it was later withdrawn.

He told House investigators that Mr Trump's delay in meeting Mr Zelensky and the decision to halt military aid deeply concerned Ukrainian officials, who view Washington as a critical ally against Russia, the people familiar with his testimony said.

Mr Volker said on Thursday that he was never given an explanation about the aid suspension, which analysts called striking.

"Volker is telling us that he spent two-plus years of his life pushing a policy line on Ukraine very energetically yet, at a critical moment in that country's political life, he had little or no visibility about what the president was actually up to, the information Trump was relying on, or even what Trump was trying to accomplish," Mr Weiss said.

Ukraine's former Foreign Minister Pavlo Klimkin rejects claims by Donald Trump concerning Joe Biden

Democrats left the deposition arguing that Mr Volker's testimony only further confirmed the damage Mr Trump and Mr Guiliani did to US foreign policy on behalf of the president's political interests.

Mr Volker, they said, made clear that the Ukrainians were confused and upset by the administration's decisions to delay the diplomatic visit and stall the military aid - and that they did not know how to handle the situation.

"I walk away very bothered by the fact that a private citizen, albeit the attorney to the president, is roaming around another country purporting to wear a semi-official hat and explicitly trying to dig up dirt on domestic political opponents," said representative Gerald Connolly.

Republicans said that Mr Volker's testimony fell far short of revealing the "quid pro quo" narrative promoted by representative Adam Schiff, who as head of the House Intelligence Committee is leading the impeachment inquiry along with the heads of the House Foreign Affairs and Oversight committees.

"Nothing he has said supports the narrative you've been hearing from Mr Schiff and the Democrats, nothing," Republican representative Jim Jordan, one of Mr Trump's top allies on Capitol Hill, said after more than two hours of Mr Volker's testimony.

Throughout Mr Volker's interview, Mr Giuliani continued to tweet images of text messages he exchanged with the former special envoy, posts attempting to counter criticisms that he was conducting rogue US foreign policy in his capacity as Mr Trump's personal attorney.

"Volker reached out to me," Mr Giuliani tweeted.

In advance of his appearance on Thursday, Mr Volker had turned over a number of documents to congressional staffers including chains of text messages with Mr Giuliani and other State Department officials, said people familiar with the documents.

On Thursday, Fox News and ABC News each obtained text messages appearing to show a top US diplomat to Ukraine, Bill Taylor, expressing concern that the Trump administration was trying to carry out a quid pro quo. "I think it's crazy to withhold security assistance for help with a political campaign," read the message.

The exchange took place in September. The group text included Mr Volker and Gordon Sondland, the US ambassador to the EU, who denied that a quid pro quo was in the works, according to Fox News and ABC News.

"Bill, I believe you are incorrect about President Trump's intentions," Mr Sondland said in a text.

"The President has been crystal clear: no quid pro quo's of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign."

Mr Volker started his job at the State Department in 2017 in an unusual part-time arrangement that allowed him to continue consulting at BGR, a powerful lobbying firm that represents Ukraine and the US-based defence firm Raytheon.

During his tenure, Mr Volker advocated for the US to send Ukraine Raytheon-manufactured antitank Javelin missiles - a decision that made the missile firm millions of dollars. BGR has said Mr Volker recused himself from all Ukraine-related matters in response to criticisms about conflicts of interest.

On Thursday afternoon, the State Department announced that it had approved the sale of 150 additional Javelin missiles to Ukraine for up to $39.2m and notified Congress.

Mr Volker also kept his job as executive director of the McCain Institute, an affiliation that may explain why Mr Volker never penetrated Mr Trump's inner circle, given the president's open disdain for the late senator John McCain.

Previously, he served as the US ambassador to Nato during the George W Bush administration.

In late July, a day after Mr Trump's call with Mr Zelensky, the whistleblower says Mr Volker and Mr Sondland held a meeting with Mr Zelensky and gave him advice about how to "navigate" Mr Trump's request.

The White House denies the charge of a "quid pro quo" with Ukraine and says that Mr Trump withheld aid to Ukraine out of frustration over Europe's lack of support for the country and its continued problems related to corruption.

When asked whether he thought anything improper occurred on the phone call, Mike Pompeo, the secretary of state, said on Wednesday that everything the Trump administration has done related to Ukraine has been "remarkably consistent" and focused on confronting the "threat that Russia poses" and rooting out "corruption" in Ukraine.