WASHINGTON — Gordon D. Sondland, the diplomat at the center of the House impeachment inquiry, kept Secretary of State Mike Pompeo apprised of key developments in the campaign to pressure Ukraine’s leader into public commitments that would satisfy President Trump, two people briefed on the matter said.

Mr. Sondland informed Mr. Pompeo in mid-August about a draft statement that Mr. Sondland and another American diplomat had worked on with the Ukrainians that they hoped would persuade Mr. Trump to grant Ukraine’s new president the Oval Office meeting he was seeking, the people said.

Later that month, Mr. Sondland discussed with Mr. Pompeo the possibility of pushing the Ukrainian president, Volodymyr Zelensky, to pledge during a planned meeting with Mr. Trump in Warsaw that he would take the steps being sought by Mr. Trump as a way to break the logjam in relations between the two countries, the people said.

Mr. Pompeo expressed his approval of the plan, they said, but Mr. Trump later canceled his trip to Poland.

The disclosures link Mr. Pompeo more directly to the Trump administration’s pressure campaign on Ukraine. It is not clear how specific Mr. Sondland, the ambassador to the European Union, was in his communications with Mr. Pompeo about what was being asked of the Ukrainians.

But Mr. Pompeo was among those who had listened in on a call between the two leaders on July 25, when Mr. Trump explicitly asked Mr. Zelensky for investigations into former Vice President Joseph R. Biden Jr. and into a debunked conspiracy theory about Ukrainian interference in the 2016 presidential campaign. A lawyer for Mr. Pompeo declined to comment.

Mr. Sondland is scheduled to testify on Wednesday in the impeachment proceedings against Mr. Trump, and he is expected to face tough questioning about gaps and misleading statements in the deposition he provided the committee last month. Mr. Sondland is sure to be grilled in particular about his failure to tell the committee about a phone call on July 26 with the president in which Mr. Trump, according to another American diplomat who overheard the call, asked Mr. Sondland if Mr. Zelensky had agreed to the investigation of Mr. Biden.

Mr. Sondland later told the diplomat, David Holmes, that when it came to Ukraine, Mr. Trump was only interested in “big stuff” that would benefit him, like the “Biden investigation” that his personal lawyer Rudolph W. Giuliani was pushing for.

Mr. Sondland’s exchanges with Mr. Pompeo suggest that he could use his testimony to counter the testimonies of other administration officials, who have said that Mr. Sondland was part of a team operating outside of normal foreign policy and national security channels that sought to do the bidding of Mr. Trump and Mr. Giuliani.

In August, Mr. Sondland and the special envoy to Ukraine, Kurt D. Volker, were in negotiations with a top Ukrainian official, Andriy Yermak, about a public statement making a commitment to investigating Mr. Biden and the energy company Burisma, which had placed Mr. Biden’s son Hunter Biden on its board. Mr. Giuliani had been pressing for that commitment and for Mr. Trump’s request that Mr. Zelensky’s government look into whether Ukrainians, not Russians, were behind the hacking of the Democratic National Committee in 2016.

Mr. Sondland and Mr. Volker have testified that they sought to get the Ukrainians to release the statement in order to satisfy Mr. Giuliani and, by extension, Mr. Trump, and to reset relations between the two countries. The Ukrainians never did it.

Mr. Pompeo has said little publicly about what he knew about the pressure campaign on Ukraine, but he has publicly criticized the Democrats’ impeachment investigation, claiming that it has been unfair to Mr. Trump and the State Department. The secretary of state had acceded to Mr. Trump’s order in the spring that he recall the United States ambassador to Ukraine, Marie L. Yovanovitch, who had been the target of a campaign of criticism involving Mr. Giuliani and two of his associates.

While Mr. Pompeo heard Mr. Trump make his demands for the investigations into the Bidens and the 2016 election on the July 25 call with Mr. Zelensky, it is not clear what he knew, or when, about the freeze over the summer of $391 million in United States military aid to Ukraine.

After a meeting on Sept. 1 between Mr. Pence and Mr. Zelensky in Warsaw, Mr. Sondland told Mr. Yermak that the resumption of the aid was tied to an agreement by Ukraine to make a commitment to the investigations sought by Mr. Trump, according to testimony made to the House impeachment inquiry.

Mr. Sondland is one of the few witnesses who spoke directly with Mr. Trump about Ukraine, making his testimony especially important for Democrats. Republicans are expected to try to undercut his credibility by laying out an array of discrepancies in it.

Mr. Sondland initially told the committee that he believed there was no link between the investigations Mr. Trump wanted and the release of the military aid. But two weeks after he was deposed, Mr. Sondland amended his testimony, and said the reverse.

After he did that, new witnesses came forward to describe events Mr. Sondland had not told the committee about. Among them was Mr. Holmes, a State Department staff member in Kyiv, Ukraine’s capital, who said in his deposition to the House Intelligence Committee that he overheard the call Mr. Sondland had with Mr. Trump in July after Mr. Sondland met with Mr. Zelensky.

“President Zelensky ‘loves your ass,’” Mr. Holmes said he heard Mr. Sondland tell Mr. Trump.

“I then heard President Trump ask, ‘He’s going to do the investigation?’” Mr. Holmes said.

“‘He’s going to do it,’” Mr. Holmes said Mr. Sondland responded.