Alex Kunhardt sent the letters to Private Rob Jacobs for a social studies assignment. Pte Jacobs, who is serving 10 miles from the North Korean border, said his excitement at getting the letters from the Brooklyn schoolchildren turned to shock as he read them.

One of the letters from the 11- to 12-year-old pupils, stamped with a smiley face, said the soldier might have been risking his life for his country, but then asked: "Have you seen how many civilians you or some other soldier killed?"

Another read: "I feel that you are being forced to kill innocent people. Iraq never attacked us, if Bush cared so much about this country then we would be out there trying to find Osama bin Laden. Bush calls this war the war on terrorism. What terrorism? Name one terrorist from Iraq ... I know I can't."

Most letters did include support for the troops, but few were completely uncritical. A Muslim boy wrote: "I know your [sic] trying to save our country and kill the terrorists but you are also destroying holy places like mosques."

Another stated: "Bush thinks he's brave ... in his safe little white house with as many guards as he thinks he needs." He concluded with: "By the way, when you shoot someone, is it great or horrible?"

Pte Jacobs, 20, told the New York Post: "It's hard enough for soldiers to deal with being away from their families, they don't need to be getting letters like this. If they don't have anything nice to say, they might as well not say anything at all." Pte Jacobs added that the letters were demoralising.

Mr Kunhardt apologised this week. "It was never my intention to demean or insult anyone. I never meant for the words of my students to hurt any of our troops," he said.

Pte Jacobs's father accepted the apology on his son's behalf.

Michael Bloomberg, New York's mayor, said: "We have freedom of speech and you certainly cannot go around censoring what people want to write. I think most of [the soldiers overseas] believe that the freedoms we have ... are protected by them putting their lives at risk."