The head of the UN children's agency voiced opposition on Thursday to the planned trial at the Guantanamo naval base of a Canadian accused of killing a US soldier while a minor in Afghanistan.

The trial of Omar Khadr, seized at an alleged al Qaeda compound in Afghanistan in 2002 when he was 15, could set a "dangerous international precedent," Anthony Lake, a former US national security adviser, said in a statement distributed by UNICEF.

Khadr, a Toronto native who was held at the Bagram air base in Afghanistan and later at the U.S. base at Guantanamo Bay, Cuba, is scheduled to go to trial in July on charges of murdering a US soldier with a grenade.

He would be the first person prosecuted in a US military tribunal for acts allegedly committed as a minor and the first to be tried at Guantanamo since President Barack Obama ordered the prison shut down after taking office in January 2009.

Obama's order has been stymied by political opposition.

Lake, who served under former U.S. President Bill Clinton in the 1990s, said UNICEF was concerned about the trial and that the Guantanamo court was not equipped to try Khadr.

"The recruitment and use of children in hostilities is a war crime, and those who are responsible -- the adult recruiters - should be prosecuted," Lake said. "The children involved are victims, acting under coercion."

Anyone prosecuted for offenses they allegedly committed as a child should be treated in accordance with international juvenile justice standards providing special protections, Lake said.

"Omar Khadr should not be prosecuted by a tribunal that is neither equipped nor required to provide these protections and meet these standards," he added.

"The prosecution of Omar Khadr may set a dangerous international precedent for other children who are victims of recruitment in armed conflicts," Lake said.

Canadian Prime Minister Stephen Harper has refused to press for Khadr's repatriation despite pressure to do some from Canadian opposition parties.