In one of the insurgent assaults, seven Americans were killed while patrolling in armored vehicles, U.S. forces spokesman Lt. Col. Todd Vician said. He said an Afghan civilian died in the same attack. The eighth American was killed in a separate attack elsewhere in the south, also while patrolling in a military vehicle, he said.
The military issued a statement saying the deaths occurred during "multiple, complex" bomb strikes. It said several troops were wounded and evacuated to a nearby medical facility, but gave no other details.
Capt. Adam Weece, a spokesman for American forces in the south, said both attacks occurred in Kandahar province. In Washington, a U.S. defense official said at least one was followed by an intense firefight with insurgents who attacked after an initial bomb went off. The official spoke on condition of anonymity because he wasn't authorized to release the information.
The deaths bring to 55 the total number of American troops killed in October in Afghanistan. The previous high occurred in August, when 51 U.S. soldiers died and the troubled nation held the first round of its presidential elections amid a wave of Taliban insurgent attacks.
The deadliest month of the Iraq conflict for U.S. forces was November 2004, when 137 Americans were killed during the assault to clear insurgents from the city of Fallujah.
"A loss like this is extremely difficult for the families as well as for those who served alongside these brave service members," said Navy Capt. Jane Campbell, a military spokeswoman. "Our thoughts and prayers are with the families and friends who mourn their loss."
The loss of life followed one of the worst days of the war for U.S. forces in Afghanistan since they launched air strikes in 2001 to oust the Taliban from power.
On Monday, a U.S. military helicopter crashed returning from the scene of a firefight with suspected Taliban drug traffickers in western Afghanistan, killing 10 Americans including three DEA agents. In a separate crash the same day, four more U.S. troops were killed when two helicopters collided over southern Afghanistan.
U.S. military officials insisted neither crash was the result of hostile fire, although the Taliban claimed they shot down a U.S. helicopter in the western province of Badghis. The U.S. did not say where in western Afghanistan its helicopter went down, and no other aircraft were reported missing.
Those casualties marked the Drug Enforcement Administration's first deaths since it began operations here in 2005. Afghanistan is the world's largest producer of opium -- the raw ingredient in heroin -- and the illicit drug trade is a major source of funding for insurgent groups.