LAHORE, Pakistan — At least seven people were killed and more than 70 wounded when a powerful explosion ripped through an Islamic religious school in northwestern Pakistan on Tuesday, officials said.

       Classes were underway early Tuesday at the school, the Jamia Zuberia madrasa, located in a crowded suburban neighborhood outside Peshawar, when the explosion shook the compound. Officials said an improvised-explosive device was most likely used in the blast.

       No group has claimed immediate responsibility.

       The majority of those injured were taken to the nearby Lady Reading Hospital. A state of emergency was declared for the city’s other hospitals, which prepared for the crush of wounded victims.

       Television footage showed a scene of devastation at the site of the blast. The religious school was cordoned off as officials combed for forensic evidence.

       The explosion broke an extended period of relative calm in Peshawar, which for years was the scene of regular terror attacks by the Pakistani Taliban. Militant attacks have ebbed since a successful 2014 military operation in the tribal regions of the province.

       The Pakistani Taliban later on Tuesday released a statement denying its involvement in the attack and called the targeting of students a “reprehensible act.”

       The threat level was raised to “high alert” in Islamabad, Pakistan’s capital, after the blast, officials said. Prime Minister Imran Khan said he was “deeply saddened by the terrorist attack.”

       “My condolences go to the victims’ families & prayers for early recovery of the injured,” Mr. Khan wrote on Twitter. “I want to assure my nation we will ensure the terrorists responsible for this cowardly barbaric attack are brought to justice ASAP.”