PESHAWAR, Pakistan—Six senior members of the Pakistani Taliban, including prominent regional leaders, vowed allegiance to the Islamic State and its head, Abu Bakr al-Baghdadi, a potentially explosive rift within militant circles here that could lead to new violence.

“I pledge allegiance to the Caliph of Muslims…Abu Bakr al-Baghdadi,” said spokesman Shahidullah Shahid. “I will listen to and obey his every order, even if the situation is difficult, whether I like the order or not.”

Mr. Shahid added that the six men didn’t represent the Pakistani Taliban, which remains in allegiance to Mullah Mohammad Omar, the spiritual leader of the Afghan Taliban.

Mr. Baghdadi proclaimed himself Caliph of the entire Muslim world in June and demanded allegiance from all Muslims. The claim put him in direct opposition to Mullah Omar, who once provided a haven for Osama bin Laden and al Qaeda while the Taliban ruled Afghanistan.

The announcement by the six Pakistani militant leaders further underscores divisions among militant Islamist groups as the Islamic State rises. Earlier this month, Mullah Fazlullah, the head of the Pakistani Taliban, expressed support for all Islamist groups fighting in Syria and Iraq, but said he continued to pledge allegiance to the leader of the Afghan Taliban.