Eight U.S. troops who were killed while on foot patrol in southern Afghanistan Thursday were hit by two consecutive blasts.
The first explosion wounded some of the service members, while the second came as others rushed to help the injured, NATO said.
The Taliban have claimed responsibility for Thursday's attack in the southern Kandahar province.
The Kandahar attack started as the troops began to inspect a suspicious object they found while patrolling on foot in the mountainous Shorabak district, 12 miles from the Pakistan border.
Forty coalition service members have been killed this month. So far this year, 191 coalition troops have died in Afghanistan.
Insurgents declared the start of a spring offensive on May 1 against NATO and the Afghan government. The offensive could affect the size of President Barack Obama's planned drawdown of U.S. troops in July.
Gen. David Petraeus, top U.S. and NATO commander in Afghanistan, has said the size of the withdrawal will depend on conditions on the ground.