At the centre of the probe, which initially stemmed from a whistleblower's complaint, is a July 25 call in which Trump pressed Zelenskiy for his cooperation
US lawmakers leading an impeachment inquiry of President Donald Trump are expected to hear another round of crucial testimony this week, including from the acting ambassador to Ukraine who had raised concerns that security aid was being withheld to apply improper political pressure.
Veteran State Department officer William Taylor and at least four other witnesses will be the latest in a parade of career diplomats and current and former US officials to speak before congressional committees, despite the objections of a Republican White House deriding the Democratic-led proceedings as a "kangaroo court."
The fast-developing inquiry has cast a new cloud of legal troubles over Trump's presidency, contributing to unease among fellow Republicans at a time when some have already been critical of his abrupt decision to remove US troops from north-eastern Syria.
Taylor, whose closed-door testimony is set for Tuesday, could be one of the most important witnesses yet. He is expected to be followed later in the week by a senior State Department diplomat, White House officials and a Pentagon Ukraine policy expert, according to an official working on the inquiry.
Taylor's text messages with US Ambassador to the European Union Gordon Sondland have surfaced as a central thread in the probe by House of Representatives Democrats into allegations that Trump improperly pressured Ukraine to investigate Democratic presidential rival Joe Biden and his son.
The inquiry is focused on whether Trump may have withheld $391 million in US security aid to Ukraine earlier this year in an effort to get newly elected President Volodymyr Zelenskiy to open an investigation into the Bidens and into a discredited theory that Ukraine may have meddled in the 2016 US election.
At the centre of the probe, which initially stemmed from a whistleblower's complaint, is a July 25 call in which Trump pressed Zelenskiy for his cooperation.
Trump has acknowledged many of the central facts related to the call, maintaining that none of it amounted to wrongdoing or a demand for a "quid pro quo," a Latin phrase meaning a favour in exchange for a favour.
"This Scam going on right now by the Democrats against the Republican Party, and me, was all about a perfect phone call I had with the Ukrainian President," Trump said on Twitter on Sunday night. "He’s already stated, NO PRESSURE!"
The impeachment inquiry could lead to the Democratic-led House passing formal charges - known as articles of impeachment - that would prompt a trial in the Senate on whether to remove Trump from office. The Senate is controlled by Trump's fellow Republicans, who have shown little inclination toward removing him.