Kamal Hyder, Al Jazeera's correspondent in Islamabad, said that local reports confirmed that there had been an incident at the camp.
"Paramilitary forces have been under a lot of pressure," he said.
"So it is possible that they may have withdrawn tactically knowing that the odds at this moment are against them."
The army said that troops at another fort in Ladah in South Waziristan did exchange fire with the fighters.
About 15 Pakistani soldiers were still missing on Thursday after hundreds of tribal fighters armed with rockets attacked Sararogha Fort, a paramilitary camp, in the country's northwest.
Seven soldiers and up to 50 tribal fighters died in the fighting on Wednesday, security officials said.
"About 700 militants attacked the fort at Sararogha at about 9:30pm (1630 GMT) and communication with the fort was cut at around 2am," a security official said.
A military official said that tribal fighters have since withdrawn from the fort.
"Miscreants have withdrawn from Sararogha Fort. Five more frontier corps troops have reached a nearby village," a brief military statement said on Thursday.
A senior military official told the AFP news agency that troops used artillery fire during the night to drive the tribal fighters out of the fort.
"Now our forces are in the process of moving in to retake the fort,"the official said.
However, there was no independent confirmation that the fighters had abandoned the British colonial-era fort in the tribal zone.
Security forces have been battling al-Qaeda-linked fighters in South Waziristan for several years.
The Sararogha area is a stronghold of Baitullah Mehsud, an al-Qaeda-linked leader who the Pakistani government say was behind the assassination of Benazir Bhutto, former primer minister and opposition leader, on December 27.