Roadside bombs killed seven American troopers Monday — including five in a single blast in Kandahar — raising to more than a dozen the number who have died in the last three days.

U.S. and Afghan soldiers gather at the scene of a roadside bomb attack in Kandahar, Afghanistan, Monday. (Allauddin Khan/The Associated Press)

The spike in deaths comes as President Hamid Karzai has publicly raised doubts about the U.S. 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 theater 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 armored as other vehicles in the convoy.

Later 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.

Death toll rising

U.S. 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 U.S. 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 longtime 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 9-year war.

With death tolls rising, Karzai has become more outspoken in his criticism of the U.S.-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 harbor suspicion of outsiders. That appears at odds with the strategy pursued by the top NATO commander, Gen. David Petraeus, which calls for U.S. troops to live closer to villagers to win their trust and protect them from the Taliban.

Last week, Karzai told a group of visiting U.S. congressmen that Obama's plan to begin withdrawing U.S. troops in July 2011 had given a "morale boost" to the Taliban and that the war could not be won until insurgent sanctuaries across the border in Pakistan are eliminated.