At least four people, including three CISF personnel, were killed and three others injured when a major fire broke out in a processing plant of the Oil and Natural Gas Corporation (ONGC) in Maharashtra's Navi Mumbai township on Tuesday, an official said.

Those injured are also personnel of the Central Industrial Security Force (CISF), the official said.

The mishap, however, had no impact on the oil processing at the unit, the ONGC said, adding that the gas was diverted to its Hazira plant in Gujarat's Surat district.

It said the fire broke out in a storm water drainage unit at its Uran plant and was doused within two hours.

Following the blaze, thick smoke engulfed the area, the police said, adding that they appealed to people staying in villages in the plant's vicinity not to panic, and assured things were under control.

The victims died during a blast while they were trying to check gas leakage at the site, a senior CISF official said.

The mishap took place around 6.47 am in the ONGC's processing plant at Uran town, located around 50 km from here, a police official said.

"Four people were killed and three others injured in the mishap," a senior police official said.

The deceased included three CISF fire personnel Eranna Nayakka, Satish Prasad Kushwaha, and M K Paswan, who were trying to stop the gas leakage, and resident production superintendent C N Rao of the ONGC, CISF's Deputy Inspector General (west zone) Nilima Singh said.

The injured persons were admitted to a nearby hospital for treatment, she said.

At least 22 fire brigade tenders, including those of the ONGC, Jawaharlal Nehru Port Trust (JNPT), Reliance Group, Navi Mumbai civic body and the Maharashtra Industrial Development Corporation at Taloja, were rushed to the spot.

The ONGC on its Twitter handle said: "Fire broke out in storm water drainage in Uran Plant early morning successfully doused within two hours by fire fighting team. #ONGCs robust crisis mitigation preparedness helped put off this major fire in a very short time."

Earlier, in another tweet, it said, "A fire broke out in storm water drainage early morning 2day in Uran oil & gas processing plant.ONGC fire services & crisis management team immediately pressed in2 action. Fire is being contained. No impact on Oil processing. Gas diverted to Hazira Plant. Situation is being assessed.

The exact cause of the fire was yet to ascertained.

According to Singh, a CISF fire station was informed in the morning about gas leakage near the LPG plant of ONGC.

The CISF personnel and others then rushed to the site and detected leakage.

"They managed to close the valve and were observing if there was any other leakage, but there was a blast and all the three CISF personnel along with ONGC DGM died," she said, adding that the blast took place in the crude handling plant of the ONGC.

The CISF fire team continued with fire fighting and managed to control the blaze.

Three other CISF firepersonnel also suffered burn injuries and were hospitalised, she said.

"Because of the alertness and dedication of the CISF personnel, a major damage to the entire plant and township was averted. The CISF personnel sacrificed their lives in the line of duty," Singh said.

Meanwhile, the police said there was nothing to worry and asked people staying near the plant to stay calm.

"The situation is under control and we appeal to the villagers staying nearby not to panic," a senior police official said.

The state-owned ONGC is the largest crude oil and natural gas company in the country.