"The move to oust me is blatant interference in domestic politics by the United States," he said, terming it an attempt at "regime change".
Khan, who has already lost his parliamentary majority after allies quit his coalition government and joined the opposition, urged his supporters to take to the streets on Sunday ahead of the vote.
Hours before he spoke, the head of the army, General Qamar Javed Bajwa, had said Pakistan wanted to expand its ties with Washington.
U.S. President Joe Biden has not called Khan since taking office, but the White House has denied that it is seeking to topple him.
Relations are strained in particular over Afghanistan, where Washington accused Pakistan of backing the successful Taliban insurgency that led last year to a chaotic withdrawal of US and allied forces.
Yet while the government has pursued multi-billion dollar development deals with China, the United States' strategic rival, the army appears keen not to jeopardise relations with Washington, which has in the past supplied it with billions of dollars in military aid.
Bajwa told a security conference in Islamabad that "we share a long history of excellent and strategic relationship with the United States, which remains our largest export market".
He noted that Pakistan had long enjoyed close diplomatic and business relationships with China, but added: "We seek to expand and broaden our ties with both countries without impacting our relations with the other."
The U.S. embassy in Islamabad did not immediately reply to a request for comment.