The explosion took place at around 11 am near Bhuna when the vehicle was on its way from Jakhal to Fatehabad, following which panic-stricken passengers jumped off to save themselves.
Fatehabad SP O P Narwal said the injured had been taken to Fatehabad hospital.
He said forensic experts have been summoned to the spot.
The incident comes two weeks after a low-intensity blast took place in a state-run bus near Pipli in Kurukshetra, leaving 12 passengers injured.
Asked if Tuesday's explosion bore any similarity to the earlier incident, Narwal said, "At this stage, we cannot comment on that. Investigations are on."
He, however, said according to preliminary accounts gathered from the spot, it appeared some passenger was carrying a chemical powder which caught fire either because of smoking or some other reason.
"It could be possible that some farmer was carrying the chemical powder, which is used in agricultural fields to scare away birds by causing an explosion," Narwal said.
The SP said no passenger had come forward to claim the chemical substance yet.
The private bus, belonging to Bhuna Friends Cooperative society, left from Jakhal at 10am for Fatehabad. Haryana has witnessed three low-intensity blasts this year, two of which occurred in trains in Panipat.
The explosion on the bus on May 26 had occurred when the vehicle was on its way from Sonepat in Haryana to Chandigarh.
Earlier, on January 16, a bomb blast had taken place on a passenger train at Panipat railway station. The explosive material was planted in a compartment next to the engine, but there was no casualty.
On May 13, an explosion had rocked an EMU train which had reached Panipat from Delhi and had been moved to the yard. Again, there was no casualty.
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