The US has made it clear that it does not side with any political party in Pakistan and supports the "peaceful" upholding of democratic, constitutional, and legal principles in the country, days after authorities slapped terrorism-related charges against ousted prime minister Imran Khan.

Khan, 69, was booked on Sunday under the Anti-Terrorism Act. On Monday, he moved to the Islamabad High Court (IHC) seeking pre-arrest bail in the case. He has secured transit bail till August 25.

Responding to a question on terrorism charge imposed on the chairman of the Pakistan Tehreek-e-Insaf (PTI) party, US State Department spokesperson Ned Price said that he was aware of the reports about the charges.

But Price said this is a matter of the Pakistani legal and judicial system.

"It is not directly a matter for the United States, 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, explaining Washington's stand on the political situation in Pakistan.

The State Department spokesperson added that the US supports the "peaceful upholding of democratic, constitutional, and legal principles in Pakistan and around the world".

Meanwhile, UN Secretary-General Antonio Guterres has also urged "calm, de-escalation of tensions" and respect for the "rule of law" in Pakistan.

In response to a question about the increasingly tense situation in Pakistan, Guterres' spokesperson Stephane Dujarric said on Monday that the UN chief was aware of the charges brought against former prime minister Imran Khan and he emphasised the need for a competent, independent and impartial legal process .

He urges calm, lowering of tensions, and respect for the rule of law, human rights and fundamental freedoms, the spokesperson added.

Khan was booked under the Anti-Terrorism Act (ATA) after he threatened an additional sessions judge, Zeba Chaudhry, and senior officers of the Islamabad Police at a rally in the federal capital's F-9 Park last week.

The Islamabad High Court has also taken notice of the threats issued by the PTI chairman and will take up a contempt of court notice to the former prime minister on Tuesday.

The political temperature in the country has been high since April when Khan was ousted from the prime minister's office. Since then, he has been rallying supporters against the government and political matters are being regularly taken up with courts.

Khan has accused the US was involved in a plot along with Pakistan's opposition parties that ousted him from power in April through a no-confidence motion. The US has repeatedly rejected Khan's allegations.