NATO officials said two coalition members were killed in a helicopter crash in eastern Afghanistan Monday.

The coalition said it is investigating the cause of the crash. Initial reports said there was no enemy activity in the area.

Earlier, another coalition aircraft also crashed in eastern Afghanistan. There were no fatalities in that crash.

Also Monday, an insurgent attack in southern Afghanistan killed a NATO service member.

NATO did not release the names or nationalities of the victims.

More than 170 NATO troops have been killed in Afghanistan this year.

In Kabul, the top U.S. commander in Afghanistan, General John Allen, led a Memorial Day ceremony for members of the NATO coalition who have died in the war.

“While our brothers and sisters fell in a place far from home, far from their families, the values for which they stood and for which they lived and for which they died occupy an enduring place in our hearts.”

On Sunday, U.S. Defense Secretary Leon Panetta defended plans to gradually wind down military operations in Afghanistan by the end of 2014.

Speaking on ABC's This Week program, the Pentagon chief said NATO forces still have a fight on their hands, but that “it is on the right track.”

Critics of a withdrawal timeline say Afghan militants will bide their time until international troops are gone.

Also Sunday, Afghan President Hamid Karzai condemned a coalition airstrike that officials say killed a family of eight in eastern Paktia province, where Afghan and international troops are trying to quell the Taliban insurgency.

A provincial government spokesman Rohullah Samoon told VOA Sunday the overnight airstrike killed a man, his wife and six children in the Gerda Serai district. He said that according to his information, the man was not linked to any insurgent group or any anti-government activities.