Police said the injured were being shifted to the District Headquarters Hospital, with police personnel also among those wounded.
Hospital administration said 25 injured people had been brought to the DHQ Hospital, where an emergency had been declared.
Police said the Kohat-Hangu Road had been closed to all traffic, while additional police personnel had reached the area near the police lines.
According to police, the blast occurred during Friday prayers. The explosion created a crater outside the mosque, severely damaged its exterior and caused a wall to collapse.
Rescue officials said nearby buildings were also damaged in the blast, while rescue and relief operations were underway at the site.
Former prime minister and Pakistan Peoples Party Central Punjab President Raja Pervaiz Ashraf expressed serious concern over the incident and called for immediate assistance for those injured.
Jamaat-e-Islami chief Hafiz Naeem ur Rehman also expressed deep grief and sorrow over the Kohat blast. He prayed for the elevation of the ranks of those who lost their lives and the speedy recovery of the injured.
Hafiz Naeem ur Rehman said the attack during Friday prayers in Kohat was a highly condemnable and cowardly act. He called on the government and administration to ensure the provision of the best possible medical facilities to the injured.
Khyber Pakhtunkhwa Information Minister Shafi Jan said the blast near the old police lines in Kohat was highly condemnable and regrettable.
According to Shafi Jan, initial reports indicated that several civilians and five police personnel had been killed and injured in the blast.
Khyber Pakhtunkhwa Chief Minister Sohail Afridi took strict notice of the incident and sought a report from the relevant authorities.
Shafi Jan said emergency had been declared at hospitals in Kohat and instructions had been issued to ensure the provision of the best possible medical treatment to the injured.
The information minister said targeting innocent citizens was an extremely regrettable and cowardly act, adding that no religion, including Islam, permits such cowardly actions.
He said such acts of terrorism could not break the nation's resolve and that terrorist elements would never succeed in achieving their objectives.
Raja Pervaiz Ashraf demanded immediate assistance for those injured in the blast.
He said those who played with human lives on the sacred day of Friday did not deserve any concession.
Raja Pervaiz Ashraf said terrorists had no religion and that the menace must be eliminated collectively.
He also called on the provincial government and law enforcement agencies to uncover the motives behind the terrorist attack.
Raja Pervaiz Ashraf said the deteriorating law and order situation in Khyber Pakhtunkhwa was a matter of concern for the entire nation. He said the Pakistan Peoples Party and its leadership stood with the victims of the blast.
