At least three people have been killed after an explosion on a train travelling between the Pakistani cities of Lahore and Karachi, officials and witnesses say.

The blast took place in an economy class train compartment on the Shalimar Express train near the town of Toba Tek Singh in Punjab province on Monday.

"The bomb, planted in one of the bathrooms, went off near the Punjab provincial district of Toba Tek Singh," Nasir Khalili, the deputy superintendent of Pakistan Railways, told Al Jazeera.

Khalili confirmed that at least three people had been killed and 12 others injured in the blast.

There was no immediate claim of responsibility for the blast.

Khalili said railway authorities had received several threats during the month of July, and had informed the security services of them.

The train service was reportedly full to capacity with passengers travelling ahead of the Muslim holiday of Eid-al-Fitr.

Television footage showed the explosion had damaged the roof of the carriage but not forced the train off the rails, suggesting a relatively low-intensity blast.