KABUL: US forces in Afghanistan have accidentally shot dead a four-year-old boy, Afghan officials said on Friday, the latest violence to strain ties between the uneasy allies

The Afghan-US relationship has been damaged by President Hamid Karzai's refusal to sign a bilateral security deal that would pave the way for a US military presence after the withdrawal of most foreign troops this year.

The United States has said its troops cannot remain without a deal. Their complete departure of US troops would leave Afghan security forces on their own to battle the Taliban.

Karzai is demanding that the United States end all unilateral military operations on Afghan territory — among other things — before the pact is signed, because they cause avoidable civilian deaths.

“We have called ... for an absolute end to ISAF/Nato military operations on homes and villages in order to avoid such killings where innocent children or civilians are the victims,” the president's spokesman, Aimal Faizi, said when commenting on the death of the boy.

The International Security Assistance force (ISAF) is Afghanistan's Nato-led force. It is dominated by US troops.

A spokesman for the governor of the southern province of Helmand told Reuters that US Marines based in the violence-plagued province mistakenly shot the boy on Wednesday because visibility was poor.

“As the weather was dusty, the Marine forces based there thought he was an enemy and opened fire. As a result of mistaken fire, he was killed,” the spokesman, Omar Zwak, said by telephone.

A spokesman for the Nato-led force said the matter would be investigated and all possible measures were taken to avoid civilian casualties.

Separately, two US service members and a civilian were killed in an aircraft crash in eastern Afghanistan on Friday,the Nato-led force said.

“At this time, there are no indications of enemy involvement in the cause of the aircraft mishap,” the force said.