"If Mullah Omar is serious about his call upon the Taliban to eliminate acts against civilians, he too should condemn this publicly," Allen said.
Separately, NATO said that two of its service members were killed, raising to 495 the number of coalition troops killed in the country so far this year. One was killed in an insurgent attack in the south on Saturday, and one died in another attack by militants Sunday in the west. NATO provided no other details.
As the U.S.-led coalition and its Afghan partners have focused their operations on Taliban strongholds in the south and east, the insurgency has carried out an increasing number of attacks in the north and west.
Shortly before the morning attack, Karzai greeted Afghans on the holiday. Breaking with past speeches marking the occasion, he made no mention of reconciliation with the Taliban and did not call on its leaders to break from the insurgency.
Ethnic minorities, who reside outside southern Afghanistan, where the Taliban are at their strongest, are the most resistant to efforts to reconcile with the insurgents.
Minorities worry that Karzai, a Pashtun, will make too many concessions to the Taliban to shore up his Pashtun base in crafting a peace deal to end the war. Assassinations of prominent northerners are likely to erode their already minimal appetite for a peace settlement.
Five leaders affiliated with the Northern Alliance, a coalition mostly composed of non-Pashtun minorities which has fought the Taliban since 1996, have been killed in a little more than a year.
Later Sunday, Karzai met with Australian Prime Minister Julia Gillard, who made an unannounced trip to the country.
Gillard's trip comes after an Afghan National Army soldier opened fire during a parade at a base in southern Kandahar province on Oct. 29, killing three Australian soldiers and wounding seven others. Australia has about 1,500 troops in Afghanistan. The attack brought Australia's death toll from the conflict to 32.