US HOUSE of Representatives Speaker Nancy Pelosi accused President Donald Trump of threatening national security amid growing calls for his impeachment for allegedly pressing Ukraine’s government to investigate a political rival.

Ms Pelosi was speaking after the president told the United States mission to the United Nations that the whistleblower who leaked details of a phone conversation with his Ukrainian counterpart Volodymyr Zelensky was guilty of espionage.

Mr Trump said: “I want to know who’s the person who gave the whistleblower the information because that’s close to a spy,” implying that revealing details of the call was an act of treason, which carries the death penalty.

US media outlets have reported that the unnamed source of the leak is a CIA officer.

The whistleblower claimed that the White House tried to lock down the details of the telephone call.

Pressure continues to mount on Mr Trump over allegations that he urged Mr Zelensky to investigate former US vice-president Joe Biden’s son Hunter.

Hunter Biden was appointed to the board of Ukrainian oil and gas company Burisma Holdings in 2014, shortly after the country’s elected government was toppled in the fascist-backed Euromaidan coup, which received US support.

The firm paid Hunter Biden up to $50,000 (£40,500) a month.

Burisma Holdings was investigated for suspected corruption and it is alleged that Mr Biden blocked the probe.

Mr Trump has dismissed the impeachment proceedings as a “hoax” and a conspiracy against him.

But Ms Pelosi said the impeachment inquiry would focus on allegations that the president threatened to withhold military aid to Ukraine unless his demands were met.

“I think we have to stay focused, as far as the public is concerned, on the fact that the president of the United States used taxpayer dollars to shake down the leader of another country for his own political gain,” the leading Democrat said.

She said Mr Trump’s actions had put national security at risk.

“This is about the national security of our country, the president of the United States being disloyal to his oath of office, jeopardising our national security and jeopardising the integrity of our elections,” she insisted.

Mr Biden’s Democratic presidential rivals failed to defend him against the accusations.

Massachusetts Senator Elizabeth Warren said she would consider barring the children of future vice-presidents children from accepting board positions with oversea companies.

This is the first impeachment inquiry against Mr Trump to gain the support of all the Democrats in the House of Representatives lower house of Congress.

However, impeachment is a two-stage process and while the Democrat-dominated lower house of Congress is now likely to back the demand, it faces being blocked in the Republican-majority Senate.