The leader of the impeachment of US President Donald Trump said on Tuesday that witnesses had provided substantial support for allegations that Trump illegally tried to force Ukraine to investigate his political rival, Joe Biden.

While the White House and Trump’s personal lawyer Rudy Giuliani refused to turn over subpoenaed documents on the Ukraine affair to Congress, House Intelligence Committee Chairman Adam Schiff said five witnesses had backed up allegations of misuse of power that could underpin formal impeachment charges.

Adam Schiff further said that witnesses have supplied information showing that Trump’s July 25 phone call with Ukraine leader Volodymyr Zelensky, in which Trump appeared to tie military aid to getting dirt on Biden from Kiev, was planned in advance and then followed up.

“We’ve made dramatic progress in answering some of the questions surrounding that July phone call… in which the president of the United States sought to coerce a vulnerable ally into conducting what can best be described as sham investigations involving his opponent,” Schiff told AFP.

Earlier on Monday, US former national security advisor was so alarmed by a White House–linked effort to pressure Ukraine to investigate Democrats, he told aide Fiona Hill to alert the National Security Council’s chief lawyer, Hill told House impeachment investigators in her 10-hour deposition.

Bolton also warned that President Donald Trump’s personal attorney Rudy Giuliani, who has emerged as the point man in the president’s alleged drive to pressure Ukraine to dig up dirt on Democratic presidential hopeful Joe Biden, is “a hand grenade who’s going to blow everyone up,” his aide Fiona Hill told lawmakers, New York Times reported.

On Saturday, Trump defended his personal attorney, Rudy Giuliani, who is reportedly under federal investigation over his dealings with Ukraine on the president’s behalf.

Last week, US President Donald Trump had said that he would participate in a going impeachment enquiry against him led by the House Democrats “if the rules are fair”.

Trump’s remarks came right after a day White House issued a letter that notified the Democrats of an uncooperative stance of the administration in relation to their efforts to remove the President from office.

“You have designed and implemented your inquiry in a manner that violates fundamental fairness and constitutionally mandated due process,” White House Counsel Pat Cipollone wrote in the letter to House Speaker Nancy Pelosi, House Intelligence Committee Chairman Adam Schiff, Oversight and Reform Committee Chairman Elijah Cummings and Foreign Affairs Committee Chairman Eliot Engel.

“This letter is manifestly wrong, and is simply another unlawful attempt to hide the facts of the Trump Administration’s brazen efforts to pressure foreign powers to intervene in the 2020 elections,” Pelosi said in a statement.

Earlier this month, Ukraine’s former president had said that he discussed investments with President Donald Trump’s attorney, Rudy Giuliani, in 2017, but that he never discussed Ukrainian companies with any US official.