Fahim Wazir, the chief government official in the region where the strike occurred, said at least 10 or 12 terrorists from outside Pakistan had been invited to attend a feast in the village of Damadola, near the Afghan border.
Wazir said in a statement on Tuesday that militants had taken away the bodies of the foreigners before the authorities arrived at the scene.
Pakistani investigators said on Wednesday they had found two empty graves at the site. But there was no information about the identities of the alleged insurgents.
Residents of Damadola in the Bajur tribal region, however, have reported that 18 civilians died in the attack, for whom as many graves were dug, and that no militants were in the area.
A senior security official, citing a report by intelligence officials in the region, said: "The residents dug 18 graves but buried 16 people and two graves were left vacant before they covered them over."
US officials have said the airstrike on Friday was meant to kill Ayman al-Zawahri, deputy to Osama bin Laden in al-Qaida.
According to intelligence sources, the CIA believed that al-Zawahri was among the foreigners, but Pakistani intelligence officers say he did not attend the feast, although he had been invited.
Pervez Musharraf, the president of Pakistan, is having to cope with protests over the deaths.
Pakistan lodged a protest with Ryan Crocker, the US ambassador, on Saturday and there have been nationwide street demonstrations. Government and opposition parties have spoken against America's action.
Despite the diplomatic protest, intelligence sources say the US has Pakistan's tacit agreement to carry out such operations in the Pashtun tribal areas. According to intelligence officers in Washington, the air attack was probably carried out by Predator drone aircraft.
Many people had expected Musharraf to broach the issue in a televised address on Tuesday, but instead he talked about a dam project, earthquake relief operations and tribal militants in Baluchistan.
Earlier, Shaukat Aziz, the prime minister, called the airstrike "one unfortunate event" in a long relationship.