A U.S. army soldier, center, takes his position at the site of a suicide attack in Kabul, Afghanistan, Monday, Oct. 13, 2014. An Afghan official said a suicide bomber targeting a NATO convoy in Kabul killed one civilian and wounded three others. (AP Photo/Massoud Hossaini)

KABUL, Afghanistan (AP) A mountain ambush by Taliban fighters killed at least 14 Afghan security force troops, authorities said Monday, as villagers elsewhere in the country alleged a NATO airstrike that the coalition said targeted militants actually killed civilians.

The fighting in Sari Pul province, as well as the disputed NATO airstrike in eastern Paktia province, show the serious challenges facing new Afghan President Ashraf Ghani Ahmadzai. Former President Hamid Karzai repeatedly clashed with NATO forces over civilian casualties from airstrikes, straining relations as public anger against the coalition grew.

The ambush in Sari Pul, where Taliban fighters reportedly have been massing for days, happened Sunday in its Kohistanat district. There, militants opened fire on an Afghan Army unit heading back to the capital after several months being deployed there, killing 12 soldiers and two police officers, said Kazim Kenhan, a spokesman for the provincial police chief.

Kenhan said 13 troops and four police officers were wounded and six troops are missing after the ambush there, some 340 kilometers (210 miles) northwest of the capital, Kabul.

"A very intensive gun battle is going on right now and the casualty number might change overnight," Kenhan said Monday. "It is a mountainous area and very difficult to reach. We do need air support as we requested from the international forces, but they didn't help us."

In Paktia province, hundreds of villagers protested over their allegation that a NATO airstrike killed seven civilians in an operation NATO said killed "eight armed enemy combatants."

The protesters brought seven corpses to the governor's office there, claiming they were civilians killed Sunday during a NATO airstrike in a mountainous area on the outskirts of the city of Gardez. The villagers said the strike targeted eight people collecting firewood and left one man wounded.

"From the evidence it seems that all seven who have been killed in the airstrike of the coalition forces are civilians, but this needs to be investigated more to find out why and how this incident has happened," said Abdul Wali Sahee, deputy provincial governor of Paktia province.

Sahee said that there was a dead body of a 12-year-old boy among those brought to the provincial capital.

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Afghans allege NATO airstrike kills 7 civilians