The exchange of rocket, mortar, and gun fire marked the fifth day this month of clashes across their disputed border.
The fighting violates a cease-fire agreement brokered on May 14 by U.S. forces.
Today's clashes were in an area where a soldier from Pakistan's Frontier Corps shot dead a U.S. soldier after talks aimed at calming tensions.
A spokesman for Pakistan's military, General Wahid Arshad, accused Afghan troops of carrying out "unprovoked" attacks today using mortars and guns from their side of the border.
He said Pakistan's Frontier Corps reacted strongly.
"This morning, there was an exchange of fire at about 3:30 or 4:00 a.m.," Arshad said. "The Afghan forces fired on us suddenly. This barrage continued for one hour. At about 7 a.m., the exchange of fire began once again."
A Different View From Afghanistan
But General Sher Ahmed Kochai, head of border guards in Afghanistan's southeastern Khost Province, told RFE/RL's Radio Free Afghanistan that it was Pakistan's Frontier Corps who broke the cease-fire.
"They fired at us with heavy artillery and rockets," Kochai said. "Two of our soldiers were injured and two of our vehicles were destroyed. We don't want fighting, and we don't want this situation to deteriorate further. We have appealed to Afghan tribesman in this area not to get involved in this fighting because we are able to defend our border."
In Kabul, Afghan Defense Ministry spokesman General Zahir Azimi also accused Pakistani forces of breaking the cease-fire and starting today's fighting.
"The first [incident ] was at 4:00 this morning when two rockets were fired at our security forces," Azimi said. "Altogether, about 20 rockets were fired [by Pakistan]. And as a result, two [members of our] security forces were injured. They were transferred to the hospital and are in good condition."
Stuck In The Middle
A spokesman for the NATO-led International Security Assistance Force (ISAF) in Afghanistan, Major John Thomas, said the conflicting claims about today's clashes have been reported to an intelligence center set up last year between Afghan, Pakistani, and NATO forces. He said authorities are trying to resolve the dispute.
But reports from the area said fighting was continuing in the afternoon.
Meanwhile, Afghan officials also have accused Pakistan of starting clashes in the same area on May 13 and May 14 -- saying that Pakistani troops advanced into Afghanistan about four kilometers across the disputed border. Pakistan has denied those charges.
Islamabad also has lodged a complaint with the tripartite commission comprising Pakistan, Afghanistan, and ISAF. That commission has dispatched an investigating team to the Afghan side of the border.
Friend Of A Friend
Both Afghanistan and Pakistan are allies in the United States' declared "war on terror."
But tensions have been running high between Kabul and Islamabad since Pakistani forces became erecting a security fence along what it says is the border.
Afghanistan has never officially recognized the Durand Line, which was drawn up at the end of the 19th century when Pakistan was part of British Colonial India, as its border.
Kabul accuses Pakistan of trying to seize Afghan land with its placement of the security fence.
Rahul Bedi, a South Asia correspondent for the London-based magazine "Jane's Defence Weekly," said the fighting has exposed old rifts between Afghanistan and Pakistan over the border issue.
"As far as the alliance between Pakistan and Afghanistan is concerned, it does not exist," Bedi said. "In fact, there is bitter rivalry between the two sides and bitter enmity between the two sides. Individually, they are linked to the Americans. But the Americans have not been able to broker any kind of an alliance between the two sides, despite both [Afghan] President [Hamid] Karzai and [Pakistani] President [Pervez} Musharraf meeting in Washington a few months ago (September 2006) and agreeing to work together. That agreement has just not worked on the ground because there is far too much baggage of history and conflict that seems to be irresolvable. And this is one of the problems that the Americans and the NATO forces are going to face."
Afghanistan's central government has confirmed that at least 13 Afghans have been killed in the fighting since May 13 -- including women, children, and Afghan police. Provincial officials in Afghanistan have said the civilian death toll in Afghanistan is higher.
(RFE/RL's Radio Free Afghanistan correspondents Najib Aamir in Peshawar and Sultan Sarwar in Prague contributed to this report.)