US Secretary of State Mike Pompeo on Saturday accused members of Congress of harassing his department to obtain documents linked to an impeachment investigation against President Donald Trump.

"There's been a congressional increase that harassed and abused State Department employees by contacting them directly and seeking to have them provide documents... That belong to the State Department, that are official US government records," he said during a visit in Greece.

"That's harassment. And I'm never going to let that happen to my team." The US congressional committees leading the impeachment probe cranked up the heat on the White House this week amid mounting evidence that Trump may have illicitly used his office to enlist Ukraine President Volodymyr Zelensky's help to damage the bid of 2020 Democratic rival Joe Biden in exchange for military aid.

After dodging questions for days, Pompeo finally confirmed Wednesday that he had been on the telephone call when Trump pressed Ukraine for damaging information on Biden.

Pompeo's Democratic critics say he is now a "witness" caught in a conflict of interest that should rule him out of decisions on how the State Department deals with the investigation.

He has been accused of "stonewalling" the investigation and trying to limit what his staff could discuss if they testify.

Pompeo, who has been subpoenaed in the probe, said Saturday his department would provide "all the documents required by the law", at a press conference in Athens. The US top official was in Greece to reinforce cooperation with its NATO ally, signing an amendment to a mutual defence agreement.