Guwahati, Apr 14 (IANS): Central and state intelligence agencies have begun their probe into a major cyber-attack at the field headquarters of state-owned Oil India Ltd (OIL) in Assam's Duliajan, officials said on Thursday.

The hacker has demanded $75,00,000 as a ransom.

"The Central and Assam's intelligence agencies are now investigating the major cyber-attack in the field headquarters of OIL," a senior OIL official told IANS, refusing to disclose further details.

OIL's Manager, Security, Sachin Kumar, had filed an FIR with the police on Tuesday night, complaining that their server, network, and other related services were affected due to the ransomware attack on Sunday.

OIL spokesperson Tridiv Hazarika, however, said that exploration and production work of the company have not been affected due to the suspected cyber-attack.

OIL is India's second largest national 'Navratna' company after ONGC in terms of total proved plus probable oil and natural gas reserves.