US Defense Secretary Mark Esper said Sunday the Pentagon would cooperate with the congressional impeachment inquiry - in an apparent break with President Donald Trump's policy to thwart the probe. 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. His 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.

Democrats 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 military aid on that favor.

Democrats slapped subpoenas Monday on the Pentagon and White House, demanding documents tied to Trump's withholding of almost $400 million desperately needed by Kiev in its conflict with Russia over Moscow's 2014 annexation of Crimea. 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."

Democrats obtained a string of text messages between US diplomats that showed the Trump administration's effort to pressure Ukraine into investigating Biden and his son Hunter, and that some diplomats in the text chain expressed concern that such actions were inappropriate. On Tuesday, Trump's White House counsel Pat Cipollone sent senior Democrats an eight-page letter declaring the inquiry illegitimate. The letter has been rejected by experts as having little legal basis.

Copyright Agence France-Presse, 2019