At least 15 Western troops have been killed in Afghanistan in August. Attacks killed at least 44 U.S service members and 31 from other international military forces in July, according to military reports.
NATO declined to say exactly where the Marines were killed or immediately release other details of the attack.
Casualties among Afghans and international troops are climbing sharply as Western forces push deeper into Taliban territory ahead of Aug. 20 presidential elections. Most violence takes place in the south and east, the traditional bases of the ethnic Pashtun insurgents. But the Taliban has also been ramping up attacks in the relatively calmer west and north.
Local officials in southern Afghanistan said Thursday that roadside explosions and a U.S. airstrike had killed at least 15 people,.
The family was traveling in a tractor with a trailer through Garmser district Wednesday morning when they hit a mine laid in the road, Helmand province police chief Assadullah Sherzad said.
A spokesman for the governor of Helmand, Daud Ahmadi, said the driver of the tractor was killed along with his wife, two children and another woman. Two other women were wounded, Ahmadi said.
Afghan officials also said a roadside bomb killed five police officers and wounded three police in Helmand on Thursday.
A police chief in a neighboring province said a Western airstrike Wednesday night killed five farmers loading cucumbers into a taxi. The U.S. initially said the men were militants who had been seen placing weapons into a van. The American military said later in the day that they had been seen planting wire-controlled roadside bombs.
A NATO spokesman, U.S. Navy Chief Petty Officer Brian Naranjo, said the military wanted to fully review video taken by the helicopter that launched the airstrike before providing a complete account of the incident.
District police chief Niaz Mohammad Sarhadi said the five were farmers trying to move cucumbers from the rural Zhari district to the city of Kandahar.
It is common for farmers to work at night in southern Afghanistan's blazing summer temperatures. Insurgents also plant bombs and move weapons in darkness, although U.S. aircraft can monitor them using night-vision equipment.
"We watched the guys loading small arms into a van for an hour before firing on it," said Lt. Cmdr. Christine Sidenstricker, a U.S. spokeswoman. "Our information is that they were loading munitions not cucumbers."
Thousands of additional U.S. and British forces have moved into southern Afghanistan to secure roads and population centers ahead of the vote. The insurgents have pledged to disrupt the election and have dramatically increased their use of roadside bombs against foreign and Afghan forces.
The U.S. and NATO have said protecting civilians is their highest priority and placed new restrictions on airstrikes last month in an attempt to limit civilian casualties.
The U.S. military also reported that one of its service members had been killed Wednesday by a roadside bomb in western Afghanistan.