KABUL, Afghanistan (AP) -- An airstrike by U.S.-led forces mistakenly killed four policemen and two civilians in eastern Afghanistan, an Afghan official said Thursday.

The civilians were two brothers who were in a car that was being searched by the police at a checkpoint when the strike occurred in the Deh Yak district of Ghazni province, according to district chief Fazel Ahmad Toolwak. He said NATO troops were fighting Taliban militants about 10 kilometers (six miles) away, but those killed in the strike were not involved in that battle.

The international coalition said it was looking into the report.

Afghans in the capital of the western Farah province, meanwhile, buried their friends and loved ones killed in a massive attack on the local government center the day before. In all, 55 people were killed, including nine attackers, and more than 100 were wounded on Wednesday.

Afghans currently lead about 90 percent of military operations nationwide as U.S. and other foreign combat forces prepare to withdraw by the end of 2014, shifting to a training and advisory role.

The Afghan troops remain heavily dependent on the coalition for air support and medical evacuations in areas where the Taliban and other militants live among the population and often enjoy local support. However, anger over reports of civilian killings prompted President Hamid Karzai to order Afghan security forces to stop requesting coalition airstrikes.

A NATO spokesman, U.S. Army Maj. Adam Wojack, said the international military coalition was assessing what happened in Ghazni.

``We are aware of local reports about an alleged airstrike in Ghazni province yesterday, in which several individuals were reportedly killed,'' he said Thursday in an email, adding the coalition ``takes all allegations of this type seriously.''

According to a recent report by the United Nations, 2,754 Afghan civilians were killed last year, down 12 percent from 3,131 in 2011. But the number killed in the second half of last year rose, suggesting that Afghanistan is likely to face continued violence as the Taliban and other militants fight for control.

The U.N. said the Taliban and other insurgents were responsible for 81 percent of the civilian deaths and injuries last year, while 8 percent were attributed to pro-government forces. The remaining civilian deaths and injuries could not be attributed to either side.

The number of casualties blamed on U.S. and allied forces decreased by 46 percent, with 316 killed and 271 wounded last year. Most were killed in U.S. and NATO airstrikes, although that number, too, dropped by nearly half last year to 126, including 51 children.

In Farah, the city hospital was overwhelmed with the casualties following Wednesday's attack. Helicopters ferried some of the wounded to other hospitals in nearby areas on Thursday.

Provincial Gov. Akram Akhpelwak said two more people had died from the attack, raising the death toll to 55 -- 36 civilians, 10 Afghan security forces and nine attackers. More than 100 people also were wounded, he said.

``Farah is a city of sadness,'' one of the province's members of parliament Humaira Ayobi said in a telephone call after attending a funeral. ``The stores are closed. There's no traffic in the streets.''

The attack began Wednesday when two suicide bombers detonated an explosives-laden vehicle near the courthouse, shattering windows and devastating several buildings. Seven others jumped out of the pickup and ran toward the courthouse and attorney general's office, prompting an 8-hour gunbattle that left many buildings pockmarked from bullets and rocket-propelled grenades.

Ayobi described a terrifying scene, with people jumping from windows to escape and the attackers shooting everybody who got in their path, including nearly two dozen people who had taken refuge in a basement. She also said two judges were singled out to be killed in a separate room with their bodies burned.

The attackers were wearing military-style uniforms that are easily bought in Afghan markets and had painted a pickup in camouflage to disguise it as an Afghan National Army vehicle so it could bypass checkpoints, she said.

Local officials said Wednesday they believed the attackers were trying to break out 15 Taliban prisoners about to stand trial. But Ayobi said the initial target may have been the governor's compound until heavy security there forced the attackers to redirect themselves to the courthouse.

In other violence, police said a tanker truck that was part of a convoy carrying fuel for a U.S. base exploded in the Bagram district of Parwan province late Wednesday.

Deputy provincial police chief Ziaull Rahman Faiez said authorities were still investigating the cause. Taliban spokesman Zabiullah Mujahid claimed responsibility, saying it was an attack.