KABUL, Afghanistan — A Taliban ambush in mountainous northern Afghanistan killed at least 14 local security force troops, authorities said Monday, as troops continued to battle militants in the region.

The fighting began Sunday in the Sari Pul province, where there have been reports of Taliban fighters massing there for an operation. The initial ambush happened in the province's Kohistanat district and saw militants kill 12 soldiers and two police officers, said Kazim Kenhan, a spokesman for the provincial police chief.

Kenhan said 13 troops were wounded and six are missing from the region, some 340 kilometers (210 miles) northwest of the capital, Kabul. Fighting between Afghan forces and Taliban militants raged Monday, he said.

Meanwhile, hundreds of Afghan villagers protested Monday over their allegation that a NATO airstrike killed seven civilians, the first such incident to test Afghanistan's new president as foreign forces begin to withdraw from the country. NATO said the strike killed "eight armed enemy combatants."

Former President Hamid Karzai repeatedly clashed with NATO forces over civilian casualties from airstrikes, straining relations as public anger against the coalition grew. There was no immediate response from Afghan President Ashraf Ghani Ahmadzai.

The protesters brought seven corpses to the governor's office in eastern Paktia province, 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.

Protested villagers shouted slogans against the foreign forces that have been in the country since the 2001 U.S.-led invasion. They demanded the Afghan government prevent such attacks in their area.

In a statement, NATO said a "precision strike resulted in the death of eight armed enemy combatants."

"No civilian casualties or damage to structures occurred during this operation," NATO said.

Separately Monday, a suicide bomber targeted a NATO convoy in Kabul while another struck an Afghan army patrol in the eastern Nangarhar province, killing three civilians in the separate assaults, authorities said.

Taliban insurgents have stepped up attacks against Afghan security forces in a bid to undermine the Western-backed government as most foreign combat troops prepare to withdraw from Afghanistan by the end of the year. The Afghan government has signed a security deal with the United States and a separate agreement with NATO allowing over 10,000 foreign troops to train and advise Afghan forces after the international combat mission ends at the end of the year.