The sons of Pakistan's jailed former prime minister Imran Khan fear authorities are concealing "something irreversible" about his condition after more than three weeks with no evidence that he is still alive, one of them said.

As court-ordered prison visits stay blocked and rumours swirl about possible prison transfers, his son, Kasim Khan, told Reuters the family has had no direct or verifiable contact with Khan, despite a judicial order for weekly meetings.

"Not knowing whether your father is safe, injured or even alive is a form of psychological torture," he said in written remarks, adding that there had been no independently confirmed communication for a couple of months.