Washington (CNN) A key witness testified in public on Wednesday that he expressed concerns to Vice President Mike Pence in September that US aid to Ukraine was being tied to investigations into President Donald Trump's political rivals.

Pence has spent weeks actively working to avoid becoming ensnared in the impeachment morass, but the revelation from US Ambassador to the European Union Gordon Sondland placed the vice president ever closer to the alleged "quid pro quo" that is at the heart of the impeachment inquiry.

And it expands the network of possible officials implicated in the scheme.

The vice president's office quickly denied the two men ever discussed the topic at hand.

But in dragging Pence to the center of the inquiry, Sondland's testimony creates more questions for a vice president whose chief personality trait is loyalty to Trump.

Pence's role had already been under scrutiny after his planned attendance at Ukrainian President Volodymyr Zelensky's inauguration in May was canceled by Trump. Later, he met with Zelensky in Warsaw after Trump canceled his planned visit to monitor a hurricane in the United States.

Sondland said in his testimony he offered Pence his view ahead of the September 1 Warsaw talks.

"I mentioned to Vice President Pence before the meetings with the Ukrainians that I had concerns that the delay in aid had become tied to the issue of investigations. I recall mentioning that before the Zelensky meeting," he said.

Sondland does not describe Pence's reaction when he raised his concerns in his opening statement. But later he said Pence nodded in response.

"The vice president nodded like you know he heard what I said and that was pretty much it as I recall," he said.

A few hours later, Pence's chief of staff Marc Short said the vice president "never had a conversation with Gordon Sondland about investigating the Bidens, Burisma, or the conditional release of financial aid to Ukraine based upon potential investigations."

"This alleged discussion recalled by Ambassador Sondland never happened," Short said.

Pence role scrutinized

It was the second day in a row that Pence's role in the alleged Ukraine pressure campaign arose in the public impeachment proceedings.

A day before Sondland appeared before the committees, a State Department official assigned to Pence's office testified about her own concerns with Trump's July 25 phone call with Zelensky, which she listened to as it was happening.

Jennifer Williams said she prepared a brief on the call for Pence , and included a transcript of it in preparatory materials for the vice president's interactions with Zelensky. But she couldn't say whether he'd read them, even though Pence has been described by other witnesses as a "voracious" reader of his briefing books.

Pence has insisted previously there was no "quid pro quo" and that Trump did nothing wrong in his dealings with Ukraine. Under repeated questioning, he has refused to say whether he knew there was a link between US aid and investigations.

He's denied ever linking the issues in his own conversations with Zelensky, and told CBS last month: "I can only tell you what I know, and what I know is that the transcript of the President's call with Ukrainian President Volodymyr Zelensky shows that there was no quid pro quo."

'Challenging environment'

As the impeachment has enveloped the West Wing, Pence has worked to keep his focus elsewhere even as his aides express anxiety at the White House's handling of the matter.

"It's a challenging environment," one Pence adviser told CNN last month.

The challenge for Pence, as it's always been, is balancing the need to appear loyal to Trump with staying clear of the President's countless controversies.

At moments, Trump has made it difficult for Pence to avoid being wrapped up in the scandal. In September, Trump recommended to reporters they ask to see transcripts of Pence's phone calls with Zelensky, insisting they would help vindicate the administration.

Even as Trump has released the log of an additional phone call with Ukraine's President, the Pence transcripts remain unseen. His office has said White House lawyers are reviewing them and that they are willing to release them when cleared.

As the impeachment efforts unfolded, Pence's aides devised a strategy to keep him out of Washington. He traveled frequently in the weeks after the House launched its investigation, from touting the administration's US-Mexico-Canada trade agreement to meeting Turkish President Recep Tayyip Erdogan in Ankara.

Indeed, he was on the road Wednesday as Sondland was delivering his damning testimony. Five minutes after Sondland began reading his opening statement, Pence's helicopter landed at Joint Base Andrews ahead of a trip to Wisconsin and Indiana, his home state.