Roadside bombs have killed seven American soldiers - including five in a single blast in Kandahar - raising the number who have died in the last three days to more than a dozen.
The spike in deaths comes as President Hamid Karzai has publicly raised doubts about the US strategy in the war, saying success cannot be achieved until more Afghans are in the front lines and insurgent sanctuaries in Pakistan are shut down.
Nato gave no details of the Monday blasts except that they occurred in the south, the main theatre of the conflict, and that five were killed in a single blast.
Witnesses said the five died when a bomb struck a Humvee on a main road on the outskirts of Kandahar, the focus of an ongoing military campaign to secure the city that the Taliban used as their headquarters during their years in power. The attackers apparently targeted the Humvee because it was not as heavily armoured as other vehicles in the convoy.
Later on Monday, a pair of rockets were fired at the Kandahar offices of the United Nations mission in Afghanistan. One fell short and slightly wounded a guard. The other overshot the compound and exploded in an empty field, police said.
US death tolls for August had been running well behind those of the previous two months that set monthly records - 60 in June and 66 in July. But 14 Americans have been killed in the last three days, raising the American US toll for the month to 49, most of them in the south.
Nato commanders have warned that casualties will mount as coalition and Afghan forces enter areas that have been under long-time Taliban control. The Nato force swelled this month to more than 140,000 - including 120,000 Americans - with the arrival of the last of the reinforcements that President Barack Obama ordered to Afghanistan in a bid to turn the tide of the nearly nine-year war.
With death tolls rising, Karzai has become more outspoken in his criticism of the US-led war effort, telling recent visitors that the American counterinsurgency strategy is flawed.
Most recently, he told the visiting speaker of the German parliament that the campaign against the Taliban over the last eight years had been "ineffective apart from causing civilian casualties," according to a statement by the presidential media office.
The statement quoted Karzai as saying Afghan forces should take the lead in efforts to win support from deeply conservative Afghan villagers who harbour suspicion of outsiders. That appears at odds with the strategy pursued by the top Nato commander, Gen David Petraeus, which calls for US troops to live closer to villagers to win their trust and protect them from the Taliban.