A roadside bomb ripped through a vehicle in eastern Afghanistan, killing a woman, her two newborn babies and the children's grandmother, an official said.

The father of the twins and the vehicle's driver also were wounded in Sunday's blast in Mandozayi district, Khost province, said Jamal Arsallah, the province's governor.

The twins were born on Saturday and the family was taking them back to their village, Arsallah said.

More coverage of the struggle for stability is available in FOXNews.com's Afghanistan Center.

It was not immediately clear why the vehicle was targeted. Militants usually use roadside bombs to attack Afghan and foreign troops on patrol.

Also Sunday, two assailants on a motorbike gunned down a high school principal in southern Afghanistan, an official said.

Abdul Rahim was leaving the mosque in Changer area near the town of Lashkar Gah in Helmand province when he was shot and killed, said Sajful Maluk Noori, the province's education director. Rahim was principal of the high school in Changer, he said.

Noori said the shooting was "a terrorist act" carried out by "the enemies of Afghanistan," a term used to describe Taliban militants.

"This act will not stop our classes or our ongoing examinations," Noori said.

Taliban militants have warned teachers that they will be killed if they continue to work for the government of President Hamid Karzai. Some 20 teachers were killed in 2006.

Visit FOXNews.com's Afghanistan Center for complete coverage.