A roadside bomb struck a school van with children and teachers in Pakistan's northwestern tribal region on Tuesday, killing five people, a government administrator said, according to The Associated Press.

The bomb went off in Khar, the main town in the Bajur region bordering Afghanistan, the AP reported. Two children, two female teachers and the driver were killed, said local government administrator, Asmatullah Wazir, but it wasn't immediately clear if anyone else was in the van or if there were any survivors.

No group immediately claimed responsibility for the attack, according to the AP.

The Bjur tribal region has been the scene of several military offensives against the Pakistani Taliban and foreign militants in recent years, the AP reported.

The attack comes against the backdrop of a major opposition rally underway since the weekend in the capital, Islamabad, according to the AP.

The demonstrators demand that Prime Minister Nawaz Sharif step down over alleged vote rigging in the 2013 parliamentary elections, the AP reported.

On Tuesday, tens thousands of demonstrators who have been camped out in twin rallies in the city since Friday were planning to march toward Sharif's office in a push to topple the government, according to the AP.

The rallies are led by cricket-star-turned-politician Imran Khan and fiery cleric Tahir-ul-Qadri who have vowed to keep up the sit-ins until Sharif resigns, the AP reported.

Khan has said he and his supporters will march into the city's Red Zone, which houses embassies, government offices and the residence of the prime minister and the president, according to the AP. The government says it will not allow the protesters to enter the Red Zone.