A suicide bomber killed a dozen people, including a leading tribal elder and five children, after a meeting Wednesday in an eastern Afghan province where U.S. forces have battled insurgents in recent weeks.
"Organizers of this brutal attack showed that they fear these elders who understand them and their wrong and inhuman intentions," Karzai said in a statement. "By killing them, they want to silence the peaceful voice of the people of Afghanistan and undermine the strength of the country."
Police and local officials said it was unclear who carried out the attack in the Asmaar district of Kunar province, where the insurgency is well entrenched. A Taliban spokesman said the group was not involved, but the area is home to a number of militant groups, including the Taliban-allied Haqqani network and Hezb-i-Islami.
"This attack demonstrates how desperate insurgents are to prevent progress by targeting Afghanistan's traditional leaders and elders," said U.S. Navy Rear Adm. Harold Pittman, a NATO spokesman.
Zareen, a former mujahedin commander who fought the Soviets in the 1980s, had a good relationship with NATO forces in the area and with Afghanistan's central government, according to the chairman of the Kunar provincial council, Haji Mia Hassan. Zareen had been targeted before by local enemies, Hassan said, but had not been singled out by the Taliban or warlords.
Zareen and other tribal elders were attacked as they left a meeting; the suicide bomber mingled with them and set off his explosives, said the local police chief, Gen. Khalilullah Ziayi.
Also in the east Wednesday, a NATO soldier was killed by a roadside bomb. Military officials did not disclose the soldier's nationality or location of the blast. So far this year, at least 121 NATO troops have been killed in Afghanistan, including 85 Americans, according to icasualties.org.
Special correspondents Hashmat Baktash and Aimal Yaqubi in Kabul contributed to this report.