At least six people, including four schoolchildren, were killed when a bomb hit a school bus in Pakistan’s restive southwestern Balochistan Province on Wednesday morning, officials said, the latest outbreak of violence in a region plagued by separatist insurgency and militancy.

Sarfraz Bugti, Balochistan’s chief minister, said that, as well as the four students, the bus driver and a helper had been killed in the attack and that several others had been wounded.

“Forty-six students were on board the bus when it was targeted with a vehicle-borne improvised explosive device,” Mr. Bugti said during a news conference in the city of Quetta.

He added that militant groups had been deliberately zeroing in on easier targets, such as children, over the past several months.

The attack took place in the Khuzdar district, about 180 miles south of Quetta, the provincial capital of Balochistan, along the main highway connecting Quetta and Karachi, the capital of Sindh Province.

The school bus was transporting students from various areas to a military-run school in a high-security cantonment, according to officials.