Intelligence agents said on Friday that the nationalities of those captured were being checked out, but they included people of several nationalities.
"It is a mixed bag," an official said. "There are Chechens, Uzbeks, an Algerian and some Arab speaking nationals. We are trying to determine their exact nationalities," he added.
The official said investigators were also trying to determine the nationalities and identities of eight men killed in the operation, that led to the capture of the 18 suspects.
"Their photographs are going to be taken and shown to the captives for identification," he said.
"It is a mixed bag.There are Chechens, Uzbeks, an Algerian and some Arab speaking nationals"
Thursday's operation in South Waziristan was one of Pakistani army's fiercest exchanges with al-Qaida and the Taliban.
Two Pakistani soldiers were killed and two wounded in the fighting after the suspects put up stiff resistance.
Authorities in Wana, capital of the South Waziristan region, said they were serving written demands to tribal elders to hand over any tribesmen who had helped shelter the suspects.
Azam Khan, senior administrative officer, said tribal leaders had agreed in May to punish any tribesmen caught harbouring al Qaida or Taliban fighters.
Many people in Pakistan's fiercely independent tribal regions are thought to be sympathetic to Afghanistan's ousted Taliban movement.
Afghanistan has also often accused Islamabad of not doing enough to curb al Qaida and Taliban members from using Pakistan as a safe haven.