WASHINGTON (REUTERS) - A former aide to US President Donald Trump's ousted national security adviser, John Bolton, will not appear before the Democratic-led congressional impeachment inquiry on Monday (Oct 28) as he awaits a court ruling, lawmakers said.

Charles Kupperman, a former deputy national security adviser, wants a court to rule on whether he should comply with a congressional subpoena or honour the Trump administration's order not to testify, his lawyers said last week.

Kupperman was compelled to appear by a lawful subpoena and US House of Representatives Intelligence Committee Chairman Adam Schiff urged witnesses such as Kupperman to "do their duty and show up."

Kupperman filed a lawsuit in a federal court in Washington on Friday naming Trump and House Democrats as defendants seeking "a declaratory judgment from this Court as to whether he is lawfully obliged to comply with a subpoena issued by the House Defendants demanding his testimony."

US Representative Jim Jordan, the top Republican on the House Oversight and Reform Committee, said Kupperman had said several times he was willing to come but "it's up to the court." Schiff noted that a federal judge on Friday validated the legality of the Democratic-led impeachment inquiry and that all witnesses need to come forward when called.

Kupperman is one of several little-known US officials summoned to testify this week, while Democrats keep their eye on Bolton, a more prominent player.

Democrats running the inquiry want to hear first-hand from Bolton, who, according to the testimony of other officials, was alarmed by a White House effort to pressure the president of Ukraine to investigate a Trump political rival, Democratic former vice-president Joe Biden.