The US military says it believes that a rocket-propelled grenade has been fired at one of its helicopters delivering aid in earthquake-hit Kashmir.
The US says the incident will not affect aid flights
The US military said the CH-47 Chinook was flying near Chakothi when the incident occurred at 1345 local time.
But Pakistan believes that dynamiting taking place to clear landslides had been mistaken for firing.
It said the incident would not affect US relief operations. The US has more than 20 helicopters delivering aid.
Chakothi is 10kms from the Line of Control that divides Pakistani and Indian-administered Kashmir. Militant groups opposed to India are based in the region.
Pakistan says 57,000 people were killed in the 8 October earthquake and three million made homeless.
The US helicopters are part of a huge international aid effort trying to bring relief to victims before the harsh Himalayan winter sets in.
The US military said: "A United States Army CH-47 Chinook helicopter flying in the vicinity of Chakothi delivering relief aid to earthquake victims is believed to have been fired upon by a rocket-propelled grenade today."
It said the helicopter returned safely to base at Chaklala at 1430 and an investigation was under way.
Pakistan's military spokesman, Maj Gen Shaukat Sultan, told the AFP news agency the helicopter crew had mistaken a dynamite blast carried out to clear landslides for grenade fire.
"Our investigation revealed that it was blasting on the roadside by engineers, under way exactly at that time when the helicopters was flying over the area," Gen Sultan said.
He said the US was "satisfied with the explanation".
However, US Commander Nick Balice at the Disaster Assistance Centre public affairs office in Islamabad, said: "Our air crew is familiar with RPG fire,"
Before the incident, the US military had promised to keep flying its helicopters in northern Pakistan and Pakistan-administered Kashmir through the winter and urged other nations to continue their efforts too.
Rear Adm Mike LeFever, commander at the Disaster Assistance Centre, said: "We are not going to diminish our helicopter support. This is long-term support. We are going to be standing by our friends, and we expect the other international communities to be able to do that."
His stance was reiterated by Commander Balice after the Chinook incident. He said it would not affect flights.
Pakistan's official death toll from the quake is now 57,597, with 78,800 injured.
More than two-thirds of the casualties were in Pakistan-administered Kashmir while 18,000 died in North-West Frontier Province.
Another 1,300 people died in Indian-administered Kashmir.
The UN says about 800,000 people still have no shelter.