The United Nations (UN) chief has said he is "aware" of the recent terrorism charges against PTI chief Imran Khan and has called for an "impartial legal process".

The ex-prime minister was booked in a terrorism case on Sunday for “threatening” a female judge and senior police officers in a public rally that drew a strong reaction from the federal government.

Stéphane Dujarric, the spokesperson for Secretary General Antonio Guterres, in response to a question during a press briefing on Monday, said the UN chief was "aware" of the charges brought Imran and emphasised the "need for a competent, independent and impartial legal process".

"The secretary general urges calm, lowering of tensions and respect for the rule of law, human rights and fundamental freedoms," he said.

Separately, US State Department Spokesman Ned Price said it was also aware of reports about the charges against Imran, but added: "This is a matter for the Pakistani legal and judicial system".

"It is not directly a matter for the US, and that’s because we don’t have a position on one political candidate or party versus any other political candidate or party," Price said at a press briefing.

He insisted that the US supported the peaceful upholding of democratic, constitutional, and legal principles in Pakistan and around the world.

Charges against Imran

In his address on Saturday evening, Imran had threatened to file cases against Islamabad’s inspector general of police and deputy inspector general of police and declared: “We won’t spare you.”

He then warned the judiciary against its “biased” attitude towards his party, saying that it should brace itself for the consequences. He also warned additional district and sessions judge Zeba Chaudhry, who had approved two-day physical remand of his aide Shahbaz Gill’s on the request of the capital police, that she would also face dire consequences.

Imran was subsequently booked in a terrorism case. He had approached the Islamabad High Court on Monday seeking bail, following which the court granted the PTI chief granted protective bail for three days.

However, the IHC taking strong exception to Imran's outburst against the female judge decided to initiate contempt of court proceedings against him and constituted a larger bench to hear the matter.

On Tuesday, the IHC issued a show-cause notice to Imran and summoned him in his personal capacity on August 31.