The TTP said its fighters would support Lashkar-e-Islam (LI) in Khyber district against the military operation named Khyber-I, launched in October.
"We have instructed our armed mujahideen to join the fight along (with) the Lashkar," the TTP said in a statement.
Khyber is among Pakistan's seven semi-autonomous tribal districts near the Afghan border, hit by homegrown insurgents and foreign militants.
It is also home to several religious extremist outfits including the Al-Qaeda.
Formed in 2007, the TTP was a formidable alliance of over a dozen groups but it has been recently weakened after defections and military operations.