KABUL, Afghanistan (AP) — U.S.-led troops and Afghan forces killed nine Afghan police Sunday, calling in airstrikes and fighting on the ground for four hours after both sides mistook the other for militants, Afghan officials said.

In a separate incident, NATO said it accidentally killed at least four Afghan civilians Saturday night. A NATO soldier also was killed in the east.

The two cases of accidental killings could further undercut popular support for the government and foreign forces operating here. President Hamid Karzai has pleaded with the U.S. and other nations fighting resurgent militants to avoid civilian casualties.

In the western province of Farah near the Iranian border, a convoy of foreign forces showed up in Anar Dara district and clashed with Afghan police, killing nine of them, said provincial Deputy Governor Younus Rasuli.

He said the foreign troops had not informed local officials they were coming, and the police thought they were enemy fighters. The two sides fought from about midnight until 4 a.m. Sunday, and the foreign forces used airstrikes, Rasuli said.

The U.S.-led coalition said it was investigating the report. It said its forces, along with Afghan troops, had retaliated in defense against ‘‘a non-uniformed hostile force.’’

‘‘The combined patrol signaled their status as coalition forces, but continued to receive fire,’’ a military statement said. ‘‘Coalition forces then returned small arms fire and engaged the enemy with precision close air support.’’

In eastern Paktika province, NATO’s International Security Assistance Force said it killed at least four civilians Saturday night when its troops fired two mortar rounds that landed nearly half a mile short of their target. NATO said it was investigating whether three other civilians also were killed in the Barmal district.

‘‘ISAF deeply regrets this accident, and an investigation as to the exact circumstances of this tragic event is now under way,’’ NATO said in a statement.

The alliance said it was providing medical aid to four civilians who were wounded.

Also Sunday, a NATO soldier was killed during fighting in the eastern Khost province, the alliance said in a statement. NATO did not identify the soldier’s nationality, but most of the troops in that area are American.

Afghanistan faces intensifying militancy nearly seven years after the U.S.-led invasion of the country ousted the hard-line Islamic Taliban movement from power.

More than 2,500 people have died in insurgency-related violence this year in the country, according to an Associated Press tally of official figures. Most have been militants, but the total includes hundreds of ordinary citizens.

NATO and the coalition insist they take great precautions to avoid civilian casualties.

Commanders accuse insurgents of endangering innocent people by launching attacks from residential areas and carrying out suicide attacks that kill far more bystanders than security personnel.

In other violence, the Ministry of Defense said Afghan troops battled insurgents in the southern Kandahar province Saturday, killing 18 militants and injuring 25. They also detained 15.

In neighboring Zabul province, Afghan troops killed nine militants and wounded seven, a ministry statement said.

Neither claim could be independently verified.

In the southern province of Helmand, a mine exploded Sunday under a civilian vehicle in Gereshk district, killing three children and wounding four people, said provincial police Chief Mohammad Hussein Andiwal.

Andiwal accused Taliban militants of planting the mine on a road frequently used by Afghan and foreign troops.

On Saturday, militants attacked a police checkpoint in the same district but in the ensuing gunfight three Taliban fighters were killed, Andiwal said. No police were injured in that clash, he added.

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Associated Press writers Amir Shah and Noor Khan in Kandahar contributed to this report.