One official said communication intercepts indicated that fighters were now telling one another to move to safe places because there were more drones in the sky and there could be more attacks.
The officials said Mehsud was not among the victims.
The US is thought to have launched more than 40 missiles against targets in the border area since last August, according to a count by The Associated Press.
Washington does not directly acknowledge being responsible for launching the missiles, which kill civilians as well as armed groups and contribute to anti-US sentiment in Pakistan.
Tuesday’s attack was the fourth in two weeks against Mehsud and his followers in his stronghold of South Waziristan.
One attack on the funeral of a dead fighter killed up to 80 people.
Pakistan’s army is deploying troops in South Waziristan and launching regular air strikes of its own to try and kill or capture Mehsud, who is blamed for organising many of the suicide attacks in Pakistan over the last few years.