A minibus full of civilians struck a roadside bomb in southern Afghanistan early Sunday, and Afghan officials said six people on board were killed.

Also Sunday, the last soldiers from the 1,600-member Dutch military contingent prepared to leave the country, marking an end to the Netherlands' four-year mission in the central province of Uruzgan.

They will be replaced by American, Australian, Slovak and Singaporean forces.

German Army Brig. Gen. Josef Blotz, a NATO spokesman, told reporters in Kabul that the Dutch pullout did not show a weakening of the international coalition.

"The overall force posture of (NATO) and of the Afghan security forces is increasing," Blotz said. However, the increase in NATO troops comes primarily from a surge of U.S. forces, who have recently taken over control of key areas in Helmand and Kandahar from British and Canadian forces.

The bus blast in Kandahar happened in the Maiwand district outside Kandahar city, according to provincial spokesman Zalmai Ayubi.

A NATO patrol arrived soon after the explosion and treated the wounded at the scene, the coalition command said.

Deadly summer for troops and civilians

U.S. and NATO forces are stepping up operations against the Taliban in Kandahar and nearby Helmand province. July was the deadliest month for U.S. forces in the nearly nine-year war, with 66 troops killed. Overall NATO deaths were highest in June, with 103 troops killed.

A NATO service member died Sunday after an insurgent attack in south, the coalition said in a statement. Details were not provided.

The escalation in military operations also threatens more civilian casualties, potentially undermining support for the U.S.-led mission among Afghans as well as the public in troop-contributing nations.

At least 270 civilians were killed in the fighting in July, and nearly 600 were wounded, Interior Ministry spokesman Zemeri Bashary said.

That's a 29 per cent increase in civilian casualties from the previous month, he said.

NATO strike provokes protest

In the Afghan capital, Kabul, more than 200 demonstrators marched toward the presidential palace to protest the alleged killing of 52 civilians by a NATO rocket strike in the south.

NATO has repeatedly disputed the allegations of civilian deaths, and Blotz said Sunday that a joint assessment team has only confirmed that one to three civilians may have been killed in the attack in Helmand province's Sangin district.

Witnesses told the assessment team that six to eight people were killed, most of them insurgents, Blotz said. Aerial pictures of graves in the area before and after the July 23 incident showed only one grave site with a "small number of fresh graves," Blotz said.

Yet the Afghans gathered in downtown Kabul said they were sure the international forces were to blame. They carried photos of children allegedly killed or wounded in the strike and shouted "Death to America! Death to NATO!"

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

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

In a letter to NATO-led forces, the top U.S. and coalition commander, Gen. David Petraeus, reminded his troops they cannot succeed in turning back the Taliban without "providing (civilians) security and earning their trust and confidence."

"The Taliban are not the only enemy of the people," Petraeus said in the letter, a copy of which was obtained by The Associated Press.

"The people are also threatened by inadequate governance, corruption and the abuse of power — the Taliban's best recruiters."

Petraeus told his troops to "hunt the enemy aggressively" but "use only the firepower needed to win a fight."

"If we kill civilians or damage their property in the course of our operations, we will create more enemies than our operations eliminate," he said.

Helmand campaign progressing 'very well'

Also Sunday, the British Defence Ministry reported a British-Afghan operation to push the Taliban out of a stronghold in the central part of Helmand province was progressing "very well" as it entered its third day.

No casualties have been reported and there has been limited contact with the Taliban, a military statement said. The operation, known as "Black Prince," is directed against Taliban forces in Sayedebad.

The departure of the Dutch from Uruzgan marked the end of a mission that was deeply unpopular in the Netherlands but widely seen in Afghanistan as among the most effective. Twenty-four Dutch soldiers have been killed since the mission began in 2006.

Only about 150 Dutch fighting forces are left in the country, and they are set to leave next week, said Maj. Henk Asma, said a spokesman for the Dutch military.

Prime Minister Jan Peter Balkenende's government collapsed earlier this year over the disagreement among coalition members on whether to keep troops in the country longer. His party, the Christian Democrats, suffered heavy losses in parliamentary elections in June.

The Dutch pioneered a strategy they called "3D" in Afghanistan — defence, diplomacy and development — that involved fighting the Taliban while at the same time building close contacts with local tribal elders and setting up numerous development projects.

Dutch troops, some of them riding bicycles, mingled closely with the local population. Often, they didn't wear helmets while walking around towns and villages as a way of winning the trust of wary local tribes.

In a message to Dutch troops, Foreign Minister Maxime Verhagen said he was "deeply impressed by the professionalism and dedication" of the soldiers and Dutch civilians working on development in the region.

"The international community and NATO are helping Afghanistan to stand on its own legs so the country can defend itself against extremists who want to use it as a breeding ground for global terrorism," Verhagen said.