A blast at a market in Pakistan's eastern city of Lahore has killed at least five people and wounded 21.

The cause of Thursday's explosion was not immediately clear.

Security forces cordoned off the area and people were not being allowed to leave or enter.

"Forensic teams have arrived on the scene and we are verifying the cause of the blast," Punjab police spokesman Nayab Haider said.

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"There are casualties and injuries but we cannot confirm figures as the rescue operation is underway."

Initial reports on Pakistani television channel Geo News said a generator had exploded but reports at the scene speculated that a bomb had gone off, based on the extent of the damage.

A witness who works at a bank in the market told Reuters that his workplace was shaken by a "frightening" explosion.

"We left the building and saw that the motorbikes parked outside were on fire and all the windows in the surrounding buildings were shattered," the witness, Mohammad Khurram, said.