Pakistani troops fired on US helicopters which had crossed into Pakistan from Afghanistan today, the Pakistani military said.
In a conflicting version of the incident, the US military denied the aircraft had flown into Pakistani territory. There were no casualties nor were the helicopters damaged, Nato said.
The shooting took place amid rising tension between Pakistan and the United States - normally close allies in the US-led "war on terror" after US troops launched missile attacks and a helicopter-borne ground assault into Pakistan this month.
The Islamabad government condemned those raids and said it would not tolerate any infringement on its territory.
Pakistan said its soldiers fired warning shots at the helicopters today after they intruded into Pakistani airspace, a Pakistani military spokesman said.
"There were two helicopters from Afghanistan that crossed into Pakistani territory," spokesman Major-General Athar Abbas said. "Our soldiers fired warning shots and those helicopters returned fire and flew back."
The Pentagon denied the helicopters had crossed the border.
A US military spokeswoman said two OH-58 helicopters had been flying a routine operation and were about 11.5 km on the Afghan side of the border in Khost Province when they were fired upon. "They received small arms fire from a Pakistani military checkpoint. The helicopters didn't cross into Pakistani airspace," she said.
Neither the helicopters, light attack and reconnaissance aircraft, nor ground forces returned fire, she said.
Pakistani president Asif Ali Zardari, speaking to reporters in New York, said his country's forces had fired flares at Nato helicopters in the border area to ensure they did not cross into Pakistani territory.
"They are flares, they are flares, just to make sure that they know they have crossed the border line," Mr Zardari said.