KABUL, Afghanistan — A Taliban attack on two aid organizations last week, the deadliest episode in a recent surge of violence against humanitarian workers in Afghanistan, is a signal to many that as peace talks falter, the insurgents are lashing out against so-called soft targets.

Wednesday’s attack killed three workers for CARE, the American aid group, and at least six others, most of them civilians. Aid workers said the true death toll was 13. In either case, it was the single biggest loss of life among the country’s 2,000 nongovernmental organizations in over a year.

The bombing, which struck CARE and Counterpart International offices, came as the sixth round of peace negotiations between the Taliban and Americans limped to an end in Qatar. The Afghan government was excluded from the talks, which ended after seven fitful days with a sense of fading optimism.

The Taliban vowed that the assault on the aid groups would not be their last.

Even before the attack, casualties among aid workers had started to rise after several years of decline. Through April, five aid workers had been killed, 12 injured and 18 abducted this year in Afghanistan, according to the United Nations’ humanitarian coordinator, Toby Lanzer.