Mehsud has been blamed for a string of suicide attacks that intensified after commandos stormed the Red Mosque complex in Islamabad last July.
Al Jazeera correspondent Kamal Hyder said: "The military forces are saying they want to neutralise this area, however, the militants are warning they would take this fight to other areas of Waziristan and that would be a serious escalation."
Mehsud's men attacked and captured another fort in the region.
As fighting intensified this week, admiral William Fallon, the head of the US military's Central Command, visited Pakistan for talks on the situation with its army chief, general Ashfaq Kayani.
Fallon told reporters in Florida last week that Pakistan was increasingly willing to fight Muslim fighters and accept US support.
He said he believed Pakistani leaders wanted a "more robust" effort by US forces to train and advise their troops in counter-insurgency operations.
The US has already announced plans to step up training of Pakistan's Frontier Corps, a paramilitary force recruited from tribal lands.