While the United States has frequently been connected with missile attacks in Pakistan – usually carried out by pilotless drone aircraft – this claim, if true, would mark "a threshold being crossed," says Ikram Sehgal, editor of Defence Journal in Karachi, Pakistan.
US forces have so far not been definitively linked to any ground operations in Pakistan, and the Pakistan government has repeatedly said it will not allow such an operation – which it calls a violation of national sovereignty – to take place.
Yet Owais Ahmed Ghani, who oversees Pakistan's restive tribal areas as governor of the North West Frontier Province, said American commandos with support from three helicopter gunships attacked a Pakistani village near the Afghan border.
"At least 20 innocent civilians of Pakistan including women and children were martyred," he said.
But claims of a NATO raid could not be verified. Indeed there was some confusion within Pakistan itself as to the nature of the attack. The Pakistani defense minister, Ahmad Mukhtar, referred to the attack as an airstrike. He also speculated that the raid targeted a specific Taliban or Al Qaeda figure.
A spokesman for US forces in Afghanistan refused comment. Defense officials at the Pentagon and at US Central Command, Tampa, Fla., would not confirm the incident and also had no further comment Wednesday morning.
The US says Pakistan's tribal regions along the Afghan border have become a haven for Al Qaeda and the Taliban. It has pressured Pakistan to fight militancy in its country, while also weighing how aggressively it can carry out strikes of its own.
Even on a day when Prime Minister Yousuf Raza Gilani's motorcade was fired upon, the allegations of a US attack in South Waziristan was leading Pakistani news. "People are not going to take it lightly," says Mr. Sehgal.
Pakistanis have long resented what they see as US interference in their country, most notably in the support the US gave to a succession of military dictators, most recently former President Pervez Musharraf. Many will take this as confirmation of US meddling. The government must strongly condemn the attack if it is to retain some of the progress it has made in recent months on the security issue, says Mahmood Shah, a former secretary of the Federally Administered Tribal Areas.
Since talks with the Taliban broke down and Pakistan resumed military operations in FATA, several tribal leaders have joined the government's side, forming militias to fight the Taliban.
In the hours after the allegations of an attack, "people have already started talking against the government," Mr. Shah says, suggesting that people see the government as complicit or too weak to stand up to the Americans.
"If America starts pulling their [the government's] leg in this fashion, then it will go the same way as General Pervez Musharraf," who resigned as president last month, he adds.
The top presidential contender in Saturday's election to replace Mr. Musharraf, Asif Zardari, is "seen as supported by the US," says Sehgal.
Mr. Zardari recently called for a cease-fire in the tribal regions in observance of the Muslim holy month of Ramadan – in part to distance himself from this perception and to win votes for the election. His election seems assured, since the president is elected by provincial parliaments and the national assembly. His coalition has a majority. But the allegations of the attack could undermine what public support he has. "The timing is a bit of a surprise," says Sehgal.