ISLAMABAD, May 17 (Reuters) - Pakistani and Afghan troops traded mortar and small arms fire on their common border early on Thursday, officials said, the latest clash in a border row straining relations between the two important U.S. allies.

The pre-dawn clash broke out on the line dividing Pakistan's tribal region of Kurram and Afghanistan's Paktia province, where several people were killed in similar clashes at the weekend.

"Fire started at around 0350 (2250 GMT) and ended at 0500. Mortar and small arms were used," a Pakistani military official said. "There are no casualty reports."

Afghan Defence Ministry spokesman Zahir Azimi confirmed the clash and said two members of the Afghan force were wounded.

Troops of both countries clashed on Sunday in the same region. The Afghan Foreign Ministry said 13 people were killed in two days of fighting.

Pakistan said the weekend fighting was triggered by "unprovoked" Afghan fire.

It was the most serious incident in years between the uneasy neighbours, whose relations have for decades been soured by disagreement over their border.

It also came two weeks after Afghan President Hamid Karzai and Pakistani President Pervez Musharraf met for the first time in months and agreed to step up security cooperation.

Relations between the two leaders have deteriorated sharply over the past 18 months. They accuse each other of not doing enough to stop Taliban violence.

Karzai repeated on Wednesday that the violence in his country emanated from Pakistan, which used to be a major supporter of the Islamists.

Pakistan denies that and says the root of the Taliban problem is in Afghanistan.

On Wednesday, thousands of Afghans chanted "Death to Pakistan, Death to Musharraf", outside the Pakistani embassy in Kabul.

On Monday, a NATO soldier was killed and four wounded in an ambush while they were returing to the Afghan side of the border after meeting Pakistani counterparts in a bid to end the skirmishes.

(Additional reporting by Sayed Salahuddin in KABUL)

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