Fifteen Afghan civilians and a German soldier were hurt in the blast, near an airfield in Takhar province on Saturday afternoon as the munitions were being loaded on to a truck for disposal, said Takhar Governor Ghulam Ghaws Abubakar.
Earlier, an Afghan Defence Ministry spokesman said six civilians and two soldiers had been killed.
Lieutenant-Colonel Karen Tissot Van Patot, a spokeswoman for the NATO-led International Security Assistance Force, said two German soldiers observing the operation were killed and another wounded.
She said the cause of the incident was under investigation, but it was being treated as an accident.
"Certainly they knew at the onset that some of the ammunition was unstable; that is why they were sorting it and anticipation of it being destroyed," she said.
The incident occurred in Rustaq, a district capital about 300km north of Kabul.
Abubakar said the blast destroyed a tent belonging to the Afghan-UN Joint Electoral Management Body organising the 18 September parliamentary election, but no election workers were hurt.
Abubakar said one Afghan was seriously hurt and all the casualties had been involved in the munitions removal.
Afghanistan is awash with weapons and munitions after more than a quarter of a century of warfare. Foreign troops have been assisting in disarming factional militias and destroying stockpiles of munitions.
German Foreign Minister Joschka Fischer said the explosion showed the risk German soldiers faced in Afghanistan.
Meanwhile, Afghan forces scouring mountains in the country's south have found the bodies of another 76 suspected rebels, bringing to 178 the death toll in some of the deadliest fighting since the fall of the Taliban in 2001.
"Our forces have collected the bodies of 76 more rebels from the battlefield"
"Our forces have collected the bodies of 76 more rebels from the battlefield," Defence Ministry spokesman Zahir Marad said on Saturday, adding that the corpses had been scattered across a wide mountainous area in and around the Miana Shien district of Kandahar province.
Fifty-six suspected rebels have also been captured since Tuesday in a blistering barrage by Afghan and US forces against rebel camps, Marad said.
In other news, government and US military leaders met in Miana Shien on Saturday with about 35 tribal chiefs to urge them to help battle the Taliban.
Dozens of Afghan and US troops guarded the meeting, which took place in a tent on a deserted field surrounded by mountains, and was attended by the governors of Zabul and Kandahar provinces, a US military commander and other top officials.
Ali Khail, a spokesman for the Zabul governor, said the officials urged the tribal leaders to cooperate in "fighting off the Taliban".
About 80 rebels were still believed to be in the mountains holding out against Afghan and US-led forces.
US and Afghan troops fought
Taliban in a fresh offensive
Two Taliban commanders, Mullah Dadullah and Mullah Brader, are believed to be surrounded in the mountainous region.
Both are well-known names in the Taliban rebellion, accused of orchestrating attacks across much of Afghanistan's violence-ridden south.
Purported Taliban spokesman Mullah Latif Hakimi on Friday denied that either man was surrounded, and said the government's death toll was exaggerated.
About 465 suspected rebels have been reported killed since March, after snows melted on mountain tracks used by them.
In the same period, 29 US troops, 38 Afghan police and soldiers and 125 civilians have been killed.