KABUL, Afghanistan — An explosion killed at least 20 people and wounded dozens of others in Afghanistan’s capital Saturday, local officials said, and many of the victims were students at a girls school.

Details of the attack in front of Sayed Ul-Shuhada high school were murky. It is unclear if there were multiple explosions, if it was a coordinated attack, or involved a car bomb or a suicide vest. But ambulances raced across the city toward the site into the evening.

No group has claimed responsibility for the attack, and the number of wounded and dead is likely to increase. Streets and roads were packed Saturday as Kabul’s residents prepared for the end of the holy month of Ramadan.

Mohammad Dawood Danish, the head of the Mohammad Ali Jinnah hospital in Kabul, said that 20 bodies and more than 40 wounded people were transferred to his hospital. Most of them were students, he said.

“The health condition of a number of girls is critical,” Mr. Danish said.

The blast comes as rights groups and others have raised fears that the looming American troop withdrawal will endanger women if the Taliban widen their grip over parts of the country, given the oppressive rules they enforced under their regime in the 1990s.

It has been a particularly violent week in Afghanistan. The Taliban have started offensives in the south and north following the start of the withdrawal of U.S. and NATO troops. Last week, a car bomb in Logar, a province just south of Kabul, killed more than 20 people.

But Saturday’s attack occurred in a predominantly Shiite part of Kabul, where the Islamic State has launched attacks before on the Hazara population, an ethnic minority. In October, a suicide blast at an education center in the same neighborhood as Saturday’s attack killed at least 24 people, again many of them students.

At least 44 civilians and 139 government forces were killed in Afghanistan the past week, the highest death toll in a single week since October, according to data gathered by The New York Times.