WASHINGTON (AP) — Secretary of State Mike Pompeo acknowledged Wednesday he was on the July phone call between President Donald Trump and Ukraine's president that's at the center of the House impeachment inquiry. But Pompeo continued to push back against what he said was Democrats' "bullying and intimidation."

The Trump administration has set a defiant tone, resisting Congress' access to impeachment witnesses, even as House Democrats warned such efforts themselves could amount to an impeachable offense.

Pompeo has tried to delay five current and former officials from providing documents and testimony in the inquiry that could lead to charges against Trump. But Democrats were able to set closed-door depositions for Thursday for former special envoy to Ukraine Kurt Volker and next week for ousted U.S. Ambassador Marie Yovanovitch.

Pompeo acknowledged at a news conference in Rome on Wednesday that "I was on the phone call" on July 25 between Trump and the Ukraine president, saying that as America's chief diplomat he was well-versed in U.S. policy toward Ukraine.

But he continued to sound a defiant note on the House impeachment probe, asserting that House investigators contacted "State Department employees directly" and told them not to contact State Department lawyers for advice. He said the State Department would "do our Constitutional duty to cooperate" with Congress but wouldn't tolerate "bullying and intimidation."

The escalating exchange of accusations and warnings signaled yet another stiffening in the confrontation between the executive and legislative branches amid the Democrats' launching of the impeachment inquiry late last week. That followed a national security whistleblower's disclosure of Trump's phone call seeking help from the new Ukrainian president in investigating Democratic political rival Joe Biden and Biden's son Hunter.

On Wednesday, the State Department's inspector general is expected to brief congressional staff from several House and Senate appropriations, oversight, foreign affairs and intelligence committees on their requests for information and documents on Ukraine, according to an aide familiar with the planning. The inspector general acts independently from Pompeo.