A US soldier has been shot dead after meeting Pakistani troops at a town near the Afghan border, Pakistan's military spokesman has said.
Pakistani and Afghan troops clashed on the border over the weekend
A Pakistani soldier also died in the shooting, Maj Gen Waheed Arshad said. He blamed the attack on militants.
A Nato statement said one of its personnel had been killed but did not confirm the soldier's nationality.
Maj Gen Arshad denied Afghan government claims a Pakistani officer opened fire on the visiting US and Afghan soldiers.
Nato spokeswoman Lieutenant Colonel Maria Carl said she had little information about "who did it or anything else, other than that they received gunfire after leaving this border meeting".
Besides the Nato fatality, the alliance said two of its soldiers and two civilian employees were injured in the incident.
Representatives of the two countries and Nato were meeting to try to calm tension after cross border clashes over the weekend, the BBC's Alastair Leithead reports from Afghanistan.
The fighting had seen Afghan and Pakistani troops trading gunfire across the border, leaving several people dead, some of them civilians.
Tensions between the two neighbours are high, with Afghanistan frequently accusing the Pakistanis of failing to prevent infiltration by Taleban fighters.
US troops serving under the command of the Nato-led international peacekeeping force crossed the border into Pakistan on Monday to discuss the unrest with their Pakistani counterparts. Afghan troops went with them.
According to Maj Gen Arshad of the Pakistani military, their convoy was attacked as they left the meeting in the north-western town of Teri Mangal.
"We don't know who fired. We have ordered an inquiry. We have cordoned off the area," Maj Gen Arshad said.
He said several US and Pakistani soldiers had been hurt by gunfire.
However, an Afghan defence ministry spokesman, Zahir Azimi, said the US troops had been attacked by a Pakistani soldier.
"At the meeting, a Pakistani officer rose up and fired at US soldiers, resulting in the deaths of two soldiers and the wounding of two others," he said.
He said casualties were evacuated by US helicopter.
Maj Gen Arshad rejected the Afghan account of the incident.