KABUL, Afghanistan — Five more American troops were killed in action in Afghanistan, ending August with a spike in bloodshed that has claimed the lives of 19 U.S. service members in only four days.

The U.S. death toll for the month stood at 55 — three-quarters of them in the second half of the month as the Taliban fight back against U.S. pressure in southern and eastern strongholds. American losses accounted for more than 70 percent of the 76 fatalities suffered by the entire NATO-led force.

NATO said four of the Americans who died Tuesday were killed by a roadside bomb in eastern Afghanistan, while a fifth died in a gunfight with insurgents in the country's south. No other details were released.

The reason behind the spike in deaths is unclear because few details about the casualties are released for security reasons.

Most of the U.S. deaths occurred in the southern provinces of Helmand and Kandahar, longtime Taliban strongholds that are the focus of the American-led operation against the insurgents.

NATO and Afghan forces are ramping up operations in Afghanistan, especially in the area around Kandahar City, the Taliban birthplace.