NATO officials in Afghanistan say coalition aircraft may have mistakenly killed several children during a bombing raid last week in the eastern part of the country.

Afghan President Hamid Karzai has condemned the air strikes and ordered an investigation after saying that eight children were killed on February 8.

NATO spokesman Brigadier General Carsten Jacobson said Monday coalition aircraft and ground forces attacked insurgents in Kapinsa province. After the raid, NATO forces found several “young Afghans of varying ages” among the casualties.

Jacobson said the deaths of innocent people are a tragedy but insisted that so far, they are not yet certain how the children's deaths happened.

The issue of civilian casualties caused by coalition operations has long been a source of tension between President Karzai and NATO.

A United Nations report released earlier this month said more than 3,000 civilians were killed in 2011 — the worst death toll in the decade-long Afghan war.

Officials with the U.N. mission in Afghanistan said insurgents were responsible for 77 percent of Afghan civilian deaths and that the number of deaths caused by foreign and local forces dropped by 4 percent.