A helicopter belonging to the NATO-led International Security Assistance Force flies over Kabul as the sun sets October 26, 2006. REUTERS/Ahmad Masood

WASHINGTON (Reuters) - NATO helicopters fired upon by Pakistani forces on Thursday were U.S. military aircraft operating inside Afghanistan, the Pentagon said.

“They were U.S. helicopters,” Pentagon spokesman Bryan Whitman told reporters at a briefing. “The flight path of the helicopters at no point took them over Pakistan.”

Whitman said U.S. and NATO military officials were speaking to their Pakistani counterparts to determine what happened and to ensure there was no recurrence.

“The Pakistanis have to provide us with a better understanding of why this took place,” he said.

Frustrated by an intensifying Taliban insurgency in Afghanistan, the United States has stepped up attacks on militants in Pakistan with six missile attacks and a helicopter-borne ground assault this month.

Pakistan strongly condemned the raids and said it would not tolerate any infringement on its territory. The Pakistani army has vowed to stand up to aggression across the border.