KABUL, Afghanistan – Roadside bombs killed seven American troops Monday – including five in a single blast in Kandahar – raising to more than a dozen the number who have died in the past three days.

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 U.N. mission in Afghanistan. One fell short and slightly wounded a guard. The other overshot the compound and exploded in an empty field, police said.

Monday’s deaths raised the U.S. death toll for August to 49, still well behind the toll of the previous two months that set monthly records – 60 in June and 66 in July.