WASHINGTON — House Democrats next week plan to hear from several more officials in the White House, Pentagon and State Department in their impeachment inquiry of President Donald Trump, two congressional staffers told NBC News.

The move underscores the brisk pace of the inquiry as lawmakers focus on other officials who could have insight into allegations the administration was approaching Ukraine and its newly elected president, Volodymyr Zelenskiy.

The House committees conducting the inquiry have followed up requests to testify with subpoenas in several cases in which civil servants faced orders from political leaders at the State Department not to cooperate. The diplomats then agreed to testify, despite objections from the White House and the State Department.

The officials requested to testify before House committees include:

Philip Reeker, acting assistant secretary of European and Eurasian affairs at the State Department, who is scheduled to appear Oct. 23. George Kent, the deputy assistant secretary of state under Reeker, had dismissed right-wing media allegations against the then-ambassador to Ukraine, Marie Yovanovitch, as baseless and took his concerns to Reeker, according to documents received by the State Department's inspector general.

Suriya Jayanti, a foreign service officer at the U.S. embassy in Kyiv, is scheduled to testify Oct. 25. In her job, Jayanti focused on Ukraine's energy sector and the country's state-owned Naftogaz company. She likely will be asked about efforts by associates of the president's personal lawyer, Rudy Giuliani, or others in the administration to change the leadership of Naftogaz.

Acting Assistant Secretary of Defense for International Security Affairs Kathryn Wheelbarger testifies before the House Armed Services Committee on March 7, 2019. Susan Walsh / AP file

Kathryn Wheelbarger, acting assistant secretary of defense for international security affairs, has also been called to testify next week. House lawmakers are interested in how and when the Pentagon learned about a White House decision to hold up the release of military assistance funds for Ukraine, and what the rationale for the delay was. Laura Cooper, the deputy assistant secretary of defense covering Russia, Ukraine, Eurasia and the Western Balkans, who would have overseen security assistance funding for Ukraine, is due to testify Friday.

Alexander Vindman, director of European affairs at the National Security Council, is due to appear Oct. 24. According to testimony from Fiona Hill, a former Russia adviser at the NSC, Giuliani oversaw a "shadow foreign policy" on Ukraine for the president's personal political gain while shutting out NSC staff and career diplomats.

Timothy Morrison, who succeeded Hill at the NSC, is due to appear Oct. 25. Lawmakers likely will want to hear what transpired on Ukraine after Hill stepped down from her post, and how the "shadow" foreign policy effort continued.