Esper said his department would try to comply with a subpoena from House Democrats seeking records relating to the withholding of US military aid to Ukraine.

"We will do everything we can to cooperate with the Congress," Esper said on CBS's Face The Nation.

Esper's reassurance came days after the White House announced it would not cooperate with the Democratic-led House of Representatives, calling its impeachment push illegitimate and unconstitutional.

Trump himself has been using Twitter to bombard the public with conspiracy theories about a secret "deep state" trying to remove him from power.

Democrats slapped subpoenas Monday on the Pentagon and White House budget office, demanding documents tied to Trump's withholding of military aid to Ukraine.

The chairmen of three House panels leading the investigation are seeking information related to Trump pressing his Ukrainian counterpart to uncover dirt on 2020 US presidential contender Joe Biden -- and the alleged conditioning of aid on that favor.

The materials are "necessary for the committees to examine... the reasons behind the White House's decision to withhold critical military assistance to Ukraine that was appropriated by Congress to counter Russian aggression," they said.

Esper told Fox News Sunday that administration officials and Trump himself may place restrictions on the release of documents demanded by the subpoena.

"I don't know the status of what that document preparation is. I don't know what restrictions they will have internally," Esper said.

"The White House has a say on the release of documents as well. There are a number of things that play into this."