Suspected US commandos from Afghanistan killed 20 people, including women and children, in a pre-dawn raid inside Pakistan, officials said, an attack branded as an assault on the nation's sovereignty.
The attack is likely to spark uproar in Pakistan, where it will be seen as undermining sovereignty at a time when a new civilian government is struggling to assert authority in the turbulent nuclear-armed state.
"It is outrageous," Owais Ahmed Ghani, governor of North West Frontier province, said in a statement.
"This is a direct assault on the sovereignty of Pakistan and the people of Pakistan expect that the armed forces ... would rise to defend the sovereignty of the country and give a befitting reply," he said.
Security officials in the region said they suspected US soldiers backed by helicopter gunships mounted the attack.
A spokeswoman for Afghanistan's NATO-led force said she had no information about the incident.
A spokesman for a separate US-led coalition force declined to comment, referring questions to the US Central Command.
The United States says al-Qaeda and Taliban militants are based in sanctuaries in northwest Pakistan's ethnic Pashtun tribal areas on the Afghan border, where they orchestrate attacks in Afghanistan and Pakistan and plot violence in the West.
Since the emergence of a civilian-led government following elections in Pakistan in February, there has been mounting concern that us military operations were becoming more aggressive in the tribal lands bordering Afghanistan.
The number of missile attacks launched by pilotless drone aircraft have multiplied, and there have been fears us forces would use helicopter gunships or put troops on the ground for "hot pursuit" or commando-style raids to destroy al-Qaeda nests.
The attack took place in a village in the South Waziristan region, a known sanctuary for al-Qaeda and Taliban militants.
The village is across the border from Bermal, a village near a US base at Shikin in Afghanistan's Paktika province.
"Troops came in helicopters and carried out action in three houses," said Angor Adda village shopkeeper Gul Nawaz.
Other residents said the foreign troops detained some people and took them away.
us-operated drone aircraft have launched attacks in Pakistani border regions several times this year, killing dozens of militants, but US ground troops had not been known to cross into Pakistan to fight militants.
Pakistan is a staunch US ally, even though the campaign against militancy is deeply unpopular with many people, but it rules out any encroachment by foreign soldiers onto its soil.
Pakistani Defence Minister Ahmed Mukhtar said aircraft belonging to foreign forces in Afghanistan attacked three houses at 4.30 a.m. (2230 GMT).
He declined to comment further, saying the Foreign Ministry was investigating.
Reuters