The 16-vehicle convoy had been guarding lorries delivering food.
Intelligence officials in South Waziristan said the fighters had taken the soldiers to different hideouts in the mountains.
A fighters' leader told the Associated Press news agency: "About 300 soldiers were present in our areas. We captured them, snatched their weapons and later shifted them to different places."
"We have taken no decision to free the soldiers," he said.
Fighters in South Waziristan released 18 paramilitary troops and one civilian official this week after holding most of them for nearly three weeks.
They had earlier killed one of the soldiers.
Many al-Qaeda and Taliban supporters took refuge on the Pakistani side of the Afghan border after US forces toppled the Taliban government in late 2001.
Violence in Pakistan, mainly in Waziristan and other parts of the northwest, has escalated since the collapse of a peace deal with tribal fighters and an army crackdown on a radical mosque in Islamabad last month.
In the latest incident, fighters attacked a police post with rocket-propelled grenades in the Swat region of North West Frontier Province, killing two policemen and wounding six, the region's police chief said.
In another incident in the same region, a passerby was killed and another wounded when a roadside bomb blew up near a police vehicle.
The vehicle was damaged but the policemen were not hurt, said Mohammad Iqbal.