This article is more than 11 months old

This article is more than 11 months old

Donald Trump’s secretary of defense said on Sunday the Pentagon would cooperate with the House’s impeachment inquiry, while cautioning that Trump may try to restrict his disclosure of information.

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Mark Esper said in two interviews his department would work to comply with a subpoena from committees 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.

“We will do everything we can to respond to their inquiry,” he said on Fox News Sunday.

But Esper warned on Fox News Sunday that Trump and other officials may yet create complications for the compliance before Tuesday’s deadline for him to respond.

“I don’t know what restrictions we may have internally in regard to releasing them,” Esper said. “The White House has a say on the release of documents as well.”

I don’t know what restrictions we may have internally … the White House has a say on the release of documents as well Mike Esper

Democrats are investigating whether Trump withheld military aid worth $250m for Ukraine as leverage while urging its leader to investigate unfounded allegations of corruption against the son of former vice-president Joe Biden, the Democratic frontrunner to face Trump in next year’s election.

The Democratic chairmen of three House committees sent a subpoena to Esper on 7 October, giving him until 15 October to respond and warning that failing to do so would constitute evidence of obstruction.

The day after the subpoena was sent to the Pentagon, Trump’s White House counsel, Pat Cipollone, sent senior Democrats an extraordinary eight-page letter that made several sweeping and false claims about the impeachment inquiry’s legal basis.

Declaring the inquiry to be illegitimate, Cipollone wrote that Trump “cannot permit” his administration to comply with the inquiry and “the executive branch cannot be expected to participate in it”.

The impeachment inquiry began after a CIA whistleblower told the watchdog for the US intelligence agencies Trump withheld the delivery of military equipment to pressure Ukraine’s government to investigate Biden and his son, Hunter.

The White House then released a partial transcript of a 25 July telephone call between Trump and President Volodymyr Zelenskiy, which confirmed that Trump urged Zelenskiy to look into the Bidens and a conspiracy theory related to Russia’s interference in the 2016 election.

Our primary interest right now is to make sure the whistleblower is protected Adam Schiff

Trump denies there was a quid pro quo, despite his own White House transcript showing that when Zelenskiy raised the US military assistance, Trump said: “I would like you to do us a favour though”.

The impeachment inquiry has also obtained text messages sent between top US officials in the region. In one message, a senior US diplomat in Ukraine, William Taylor, told US ambassador to the EU Gordon Sondland: “I think it’s crazy to withhold security assistance for help with a political campaign.”

In a potential blow to Trump, Sondland – previously considered a loyalist – is reportedly preparing to testify that Trump possibly did consider the Biden investigation and delivery of the aid to be a quid pro quo.

The Washington Post reported that the ambassador will tell Congress this week that he worked to persuade Ukraine to announce an inquiry into an energy company where Hunter Biden worked, and dangled a meeting at the White House for Zelenskiy as an incentive.

Sondland reportedly intends to claim he did not realise the company was linked to the Bidens.

“It was a quid pro quo, but not a corrupt one,” the Post reported a person familiar with Sondland’s testimony had said in summarising Sondland’s position.

Facebook Twitter Pinterest Gordon Sondland, US ambassador to the European Union, seen in Bucharest. Photograph: Daniel Mihăilescu/AFP/Getty Images

Esper did not answer on Sunday when asked if Trump had ever explained to him why the president had reportedly gone against the views of Pentagon officials and withheld the aid to Ukraine, which is fighting Russian-backed separatists in the east.

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“I have nothing to share with you on this,” Esper said on Fox News Sunday.

Representative Adam Schiff, the California Democrat effectively leading the impeachment inquiry as chair of the House intelligence committee, said the whistleblower may now not be called to testify, amid concerns for his safety after Trump labelled him a traitor.

“Yes we were interested in having the whistleblower come forward,” Schiff told Face the Nation. “Our primary interest right now is to make sure that person is protected.”

A lawyer for Hunter Biden said his client would resign from his job at a Chinese company, a position that has attracted further attacks from Trump and his allies.

Biden would also not serve on the boards of foreign companies if his father is elected president, the lawyer said.