By MIRWAIS KHAN, Associated Press Writer Mirwais Khan, Associated Press Writer   – KANDAHAR, Afghanistan –

A minibus full of civilians struck a roadside bomb in southern Afghanistan early Sunday, killing six on board, officials said.

Another nine people were wounded in the explosion in Kandahar province's Maiwand district, said provincial spokesman Zalmai Ayubi.

The civilian death toll has climbed along with military casualties amid a summer push into Taliban strongholds in the southern provinces of Kandahar and Helmand. It was not clear if the bombing was targeted or if the bus had struck a bomb planted in the hope of killing NATO or Afghan troops.

To counter an influx of international forces in recent months, the Taliban have stepped up assassination campaigns against government sympathizers and bombings, as well as attacks on government or NATO forces.

Threats have also appeared more frequently in Kabul, the capital to the north. On Sunday, NATO said Afghan and international forces killed an insurgent in the city while pursuing a tip that militants were planning a double suicide bombing.

The man started firing on troops when they searched a neighborhood believed to harbor insurgents, the coalition said in a statement.

Meanwhile, more than 400 demonstrators marched toward the presidential palace in Kabul to protest the alleged killing of 52 civilians by a NATO rocket strike in the south. NATO has disputed the report of civilian deaths.

Afghan and NATO representatives are conducting a joint investigation to find out the truth about the attack in Helmand province's Sangin district, but the Afghans gathered in downtown Kabul said they were sure the international forces were guilty.

They carried photos of children allegedly wounded or killed in the strike and shouted "Death to America! Death to NATO!"

"We should not tolerate such attacks. The Americans are invaders who have occupied our country in the name of fighting terrorism," said 22-year-old Ahmad Jawed, a university student.

He said the Afghan government was equally to blame for failing to exert control over NATO troops.

"We don't have a strong enough government to protect the rights of the Afghan people," Jawed said.

The demonstrators did not threaten violence and they were guarded by Afghan police who had shut down the road for the them to hold their march.