The case has sparked a bitter row between India and Italy, which has lambasted Indian officials for keeping the marines in India for two years without charges being filed.
The Supreme Court last week ordered the government to clarify the case against the marines and set a hearing for February 18. The government said it would remove the possibility of a death penalty but would still prosecute the marines under the severe anti-piracy law.
De Mistura said that invoking the anti-piracy law effectively makes India "tantamount and equivalent to a terrorist-act country."
"The international mobilization against the concept of having a country accused of terrorism for an incident in international waters by two of its own military is going to continue," he said.
The marines, Massimilian Latorreo and Salvatore Girone, are currently on bail pending trial and are living and working at the Italian Embassy in Delhi. They were part of a military security team on a cargo ship and fired at the fishermen they said they mistook for pirates.
Italy has insisted the men be sent back to Italy while awaiting the start of the trial, but India has refused.
India's attorney general had blamed the delay on witnesses who were on board the cargo ship when the firing took place but had not returned to India to give evidence despite promising that they would do so when required by the court.