US Secretary of State Mike Pompeo has said that the Department of State will follow the law in the impeachment inquiry against President Donald Trump and defended his boss' attempts to push Ukraine to investigate former vice president Joe Biden.

Pompeo's remarks came after the State Department missed a Friday deadline to comply with a subpoena issued by House Democrats to hand over documents related to the department's dealings with Ukraine and Trump attorney Rudy Giuliani.

Democrats in the US House of Representatives are examining whether there are grounds to impeach Trump, a Republican, based on a whistleblower's account that said he asked Ukrainian President Volodymyr Zelenskiy in a July 25 phone call to help investigate Democratic political rival Biden.

Pompeo, who is travelling to Greece, told reporters that the department had sent a letter to Congress, "which is our initial response to the document request."

"We'll obviously do all the things we're required to do by law," Pompeo said in response to a question.

"I was a member of Congress once; Article I has a certain set of powers, and Article II has an obligation to make sure that we protect officials at the State Department," he added.

"And sadly, there have been congressional inquiries that have harassed and abused State Department employees by contacting them directly and seeking to have them provide documents – documents that belong to the State Department, that are official US Government records – and ask them to do so without – saying, hey, don't bother calling the State Department lawyers; just talk to us directly," he said.

"That's harassment, and I'll never let that happen to my team," he asserted.

Pompeo alleged that the previous Obama administration was not responsive to the documents being sought by the US Congress and its committees.

"I remember once when I was on that side and we were looking for documents, I remember precisely how long it took for those documents to come across. We're going to beat that. We're going to be more responsive than the Obama administration was in the years preceded this particular congress,” he said.

Responding to a question on appropriateness of the investigation against Trump, Pompeo alleged that "there's clearly politics involved" in it.

Pompeo supported the administration's demand that Ukraine open inquiries into alleged corruption that could target Biden's son and alleged interference in the 2016 US presidential election

The Trump administration, he said, was incredibly focused on making sure that they worked with Ukraine in a way that was appropriate.

"It is not only appropriate, but it is our duty to investigate if we think there was interference in the election of 2016, I think everyone recognises that governments have an obligation – indeed, a duty – to ensure that elections happen with integrity, without interference from any government, whether that's the Ukrainian government or any other," he said.

"So, inquiries with respect to that are completely important, and if we need another government's assistance, it's very reasonable to ask that government, to say, do you have any help that you can provide so we can protect the American people, so that they can vote in free and fair elections without interference from any other country," he said.

"I see (it is) our duty to engage in activity that ensures that we have fair, free elections. I only wish that the previous administration had protected our elections back in 2016," he said.

"You'll recall that the interference that took place took place under the previous administration," the top US diplomat added.

In the July 25 phone call with Ukrainian President Zelensky,  Trump urged him to look into possible interference by Ukraine in the 2016 election as well as former Vice President Joe Biden's and his son Hunter's conduct in the country.

During the call, Trump asked for a "favour" in the form of Ukraine providing information about theories related to the hacking of the Democratic National Committee server in 2016.

Democrats claim that Trump used USD 400 million in military aid as a quid pro quo for the Ukrainians to investigate a political opponent and have opened an impeachment inquiry, firing off a raft of subpoenas within a few days to top administration and White House officials.

Democrats leading the inquiry suggested they were in negotiations with Pompeo's team over the subpoena.