India will pay an extra 713 million rupees ($15.8 million) in compensation to the victims of the world's worst industrial accident that killed thousands in 1984, the government said in a statement late on Thursday.

India says about 3,500 people died when a Union Carbide plant in the central Indian city of Bhopal accidentally released toxic gases into the air, but activists say 25,000 died in the immediate aftermath and the years that followed.

The 713 million rupees is in addition to a compensation package of 6.69 billion rupees ($148 million) which the government announced in June after a public outcry over the lack of payment to victims.

The government compensation was paid after campaigners criticized what they saw as excessively lenient sentences handed down to seven Indian former employees of Union Carbide convicted of causing death by negligence.

They were each sentenced to two years in jail and fined $2,100 each. The former Indian arm of Union Carbide was found guilty of negligence and fined $10,600.

In August the Supreme Court accepted a government petition seeking harsher sentences for the seven.

The government is currently looking at whether U.S. company Dow Chemicals, which bought over Union Carbide in 1999, has liability in the accident.

Dow denies responsibility, saying it took over the firm years after Union Carbide settled its liabilities to the Indian government in 1989 by paying $470 million and after it sold off its local arm.