(Corrects day in second paragraph to Thursday from Friday)

ISLAMABAD, July 12 (Reuters) - Pakistan has lodged a strong protest with the United States over mortar fire from Afghanistan, which wounded six Pakistani soldiers, a military spokesman said on Saturday.

Six mortar shells fired from across the Afghan-Pakistani border fell near a Pakistani post on Thursday in the South Waziristan tribal region, a known sanctuary for al Qaeda and Taliban militants.

"Our troops retaliated with fire immediately," the spokesman said, adding that six Pakistani security forces personnel were injured and a number of casualities reported in Afghanistan.

He did not say if U.S.-led coalition or Afghan forces fired mortars but added that a "strong protest" had been lodged with the headquarters of the coalition forces in Kabul on Friday.

An intelligence official said on Friday that the firing from Afghanistan began after Taliban had attacked a foreign troop base on the Afghan side of the border. He has said two civilians were also wounded in the fire.

The exchange of fire between the forces fighting militants on both sides of the border comes at a time of increased tension between the two neighbours, with both complaining that violence in their own country was fuelled by the situation in the other.

Afghanistan's foreign minister has said that main factors contributing to a deterioration of Afghan security was a de facto truce in tribal areas beyond the border, a clear reference to Pakistan.

The new Pakistani government led by the party of assassinated former Prime Minister Benazir Bhutto began talks with Pakistani Taliban leader Baitullah Mehsud, based in the remote tribal region, South Waziristan, through tribal elders after it came to power in March.

Mehsud suspended talks last month after security forces launched a sweep in a tribal region against Islamist militants threatening the main northwestern city of Peshawar.

The militants had unleashed a wave of attacks across Pakistan over the past year, prompting the authorities to negotiate peace with them to end the violence.

Pakistani tribesmen have reported stepped-up patrols by pilotless U.S. drones over the border areas.

U.S. drones have struck several times in northwest Pakistan this year, killing dozens of suspected militants.

Last month, Pakistan announced its outrage when 11 of its border soldiers were killed in a U.S. air strike as U.S. forces battled Taliban militants.

Days later, Afghan President Hamid Karazi, increasingly frustrated with Taliban attacks into Afghanistan from Pakistani sanctuaries, threatened to send troops into Pakistan to fight the insurgents.

Pakistan, a key ally in the U.S.-led war on terrorism, has been battling militants in the tribal areas, but is wary of allowing foreign troops to operate on its soil out of fear of a backlash from fiercely-independent ethnic Pashtun tribes. (Reporting by Zeeshan Haider; Editing by Valerie Lee)

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