A homicide bomber apparently planning to attack a meeting of NATO and tribal officials blew himself up in a busy market district Thursday in central Afghanistan, killing at least 20 people, officials said, making it the deadliest attack against civilians in more than three months.

The attack came a day after the U.N. released a report showing that the number of Afghan civilians killed in war-related violence was at its highest level last year, and homicide bombings and other attacks blamed on insurgents were the leading cause of death.

The blast tore through shops in the town of Dihrawud, in Uruzgan province, as the area was packed with people gathered for the weekly bazaar.

Police said they believed the attacker's target was a meeting for NATO and local officials to discuss security with dozens of tribal elders. It was unclear why he detonated his explosives before reaching the heavily guarded building where the meeting was being held.

District police chief Omar Khan, who was at the meeting, said NATO forces had surrounded the building for the meeting, which is held on a regular basis.

Lt. Nico Melendez, a NATO spokesman in Kabul, said he had no indication of a NATO connection to the blast.

The conflicting accounts could not immediately be reconciled.

NATO forces responded to the blast, which occurred near the Forward Operating Base Hadrian, and said initial reports indicated at least 20 Afghan civilians had been killed and 13 wounded.

Those killed included three children, according to provincial police chief Gen. Juma Gul Himat said. Several shops were destroyed.

It was the deadliest attack since Sept. 29, when a bomb struck a crowded bus on the outskirts of the city of Kandahar.

Uruzgan is a mostly Pashtun province that saw major fighting in 2007. It also was the scene of a June 2002 incident in which U.S. aircraft mistakenly attacked a wedding party, killing more than 30 people.

Another homicide bomber targeted a police patrol Thursday in the southern town of Musa Qala, killing an Afghan national police officer and wounding four civilians, according to officials and NATO.

Provincial spokesman Daoud Ahmadi said the bomber was on foot but NATO said the attacker was in an explosives-laden vehicle.

Musa Qala is in the province of Helmand, which is expected to be a major focus for NATO forces as they step up efforts to rout the Taliban.

Underscoring the dangers, police also said Thursday that four would-be homicide bombers were killed in a premature explosion the night before near the city of Kandahar.

The suspects were traveling from the Panjway district to Kandahar when the blast occurred about 8 p.m. Wednesday, deputy provincial police chief Fazel Ahmad Sharzad said.

He said the men were planning an attack in Kandahar but explosives in the car detonated before they reached the city. He didn't say what the target may have been.

The U.N. report said that more than half the 2,412 civilian deaths last year were the result of homicide bombings and other attacks despite an order by Taliban leader Mullah Mohammed Omar to avoid endangering noncombatants.

The attacks typically target government or international forces but civilians have increasingly been caught in the middle.