Four U.S. Marines were killed when a roadside bomb struck their vehicle in western Afghanistan, driving up Western military deaths at a pace that would make August one of the deadliest months of the war.

Civilian casualties have also been sharply rising, and a southern Afghan police chief said Thursday a U.S. airstrike killed five farmers picking their cucumber crop at night. The U.S. military said the men were insurgents, but offered conflicting accounts throughout the day of the reason an American helicopter opened fire.

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.

Casualties among Afghans and international troops are climbing 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.

NATO declined to say exactly where the Marines were killed or immediately release other details of the attack.

In southern Afghanistan, thousands of additional U.S. and British forces have been trying 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.

But a police chief in the southern province of Kandahar said a Western airstrike Wednesday night killed five farmers loading cucumbers into a taxi. A U.S. spokeswoman initially said the men were militants who had been seen placing weapons into a van.

"We watched the guys loading small arms into a van for an hour before firing on it," said U.S. Lt. Cmdr. Christine Sidenstricker, a press officer for NATO forces in Afghanistan. "Our information is that they were loading munitions not cucumbers."

District police chief Niaz Mohammad Sarhadi said the five were trying to move their cucumber crop from the rural Zhari district to the city of Kandahar.

In neighboring Helmand province, local officials said roadside explosions killed at least 10 other people, including members of a family on their way to a wedding party, Helmand police chief Assadullah Sherzad said.

Afghan officials also said a roadside bomb killed five police officers and wounded three police in Helmand on Thursday.

Earlier in the day, the U.S. military reported that one of its service members had been killed Wednesday by a roadside bomb in western Afghanistan.