A demonstration has been held in south-east Afghanistan accusing U.S. troops of insulting Islam after they distributed soccer balls bearing the name of Allah.
The balls showed the Saudi Arabian flag, which features the Koranic declaration of faith.
The U.S. military said the idea had been to give something for Afghan children to enjoy and they did not realise it would cause offense.
The soccer balls were dropped from a helicopter in Khost province.
Some displayed flags from countries all over the world, including Saudi Arabia, which features the shahada, one of the five pillars of Islam -- the declaration of faith.
The words, which include the name of Allah, are revered, and Muslims are very sensitive about where and how they can be used.
Saudi Arabia has complained to the World Cup's ruling body in the past about the use of its flag on soccer balls.
Mullahs in Afghanistan criticised the U.S. forces for their insensitivity, and around 100 people held a demonstration in Khost.
Afghan MP Mirwais Yasini said: "To have a verse of the Koran on something you kick with your foot would be an insult in any Muslim country around the world."
A spokeswoman for the U.S. forces in Afghanistan said they made "significant efforts to work with local leaders, mullahs and elders to respect their culture," and distributing the footballs was an effort to give a gift the Afghan children would enjoy.
"Unfortunately," she added, "there was something on those footballs we didn't immediately understand to be offensive and we regret that, as we do not want to offend."