Breaking: Afghan, Pakistani troops exchange fire.

Afghan and Pakistani military troops exchange fire across an unmarked part of the border in an incident that left one Pakistani soldier dead.

Both sides accuse the other of initiating the conflict.

An Afghan border police commander in the eastern province of Khost confirmed the exchange of fire and accused Pakistan of opening fire first.

"At around 11:00 am (0630 GMT) today, Pakistani troops in Waziristan started firing heavy and light weapons towards police posts in Gurbuz district. Our soldiers returned fire," AFP quoted the official as saying.

"Their attack was completely unprovoked and without reason. The fighting is still continuing, there hasn't been casualties on our side," he added.

The unmarked border area between Afghanistan and Pakistan has been classified by Washington as one of the most dangerous spots on Earth and a headquarters of al-Qaeda and Taliban.

A Pakistani security official confirmed the death of one soldier and said eight others were wounded, with two of them in serious condition.

Pakistani intelligence officials claim the incident began when Afghan troops fired mortars from across the border into Bange-e-Dar checkpoint in Ghulam Khan town in North Waziristan tribal district, according to AFP.

"We fired in retaliation. Our troops are using artillery and mortars," said a senior military official in the northwestern city of Peshawar.

Afghanistan and Pakistan border troops have exchanged fire across the border in the past.

The incident was the most serious cross-border clash since May 2007, when clashes between Afghan and Pakistani troops left three civilians and a police officer dead.

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