About 16 fighters and one government soldier have been killed during a clash in Pakistan's northwestern tribal region, army officers have said.
The attack took place in the Bajaur tribal district bordering Afghanistan on Saturday morning.
Bajaur is one of seven semi-autonomous tribal regions of the Federally Administered Tribal Areas (FATA) of Pakistan.
The fighters were killed when they entered Pakistan and attacked a military checkpoint, killing one soldier and wounding two others.
The Associated Press news agency, quoting unnamed Pakistani army officers, said the military was backed by helicopter gunships. Another military official confirmed the attack and casualties, which could not be verified independently.
The military operation in the area is going on, according to reports.
This is the first attack on the Pakistani army since the Tehreek-e-Taliban Pakistan (TTP) split into factions, with the Mehsud tribesmen parting ways from the main group.
Pakistan and Afghanistan share a volatile and porous border that is often the scene of cross-border attacks. Fighters from both countries also cross the border to use the neighboring country's soil as a safe haven.
Since the TTP rose up against the Pakistani state in 2007, almost 7000 people have been killed in violent attacks around the country.