Al-Habib al-Amin, the country's culture minister and a top aide to Libya's prime minister, told reporters in a televised news conference that government forces including navy vessels were deployed to al-Sidra port to stop the tanker.
"It's final and decisive. Any attempt (by the tanker) to move, it will be turned into scrap," al-Amin said.
The Libya Revolutionary Operation Room, an umbrella group of militias groups that answer to the interim parliament, said in a statement on its official Facebook page that the tanker is at the port and "couldn't leave because our hero revolutionaries are besieging it and preventing it from leaving."
Al-Sidra is one of the biggest ports in the country and has been under militia control since the summer, slowing the country's oil output, once estimated at 1.6 million barrels a day, to a trickle.
The seizure of the terminals and attempted oil sales show Libya's security and economic woes which have piled up over the past two years since the toppling of dictator Moammar Gadhafi in 2011.
For months, the Libyan government has been coaxing and threatening to use force against the eastern militias demanding greater self-rule and equal distribution of oil wealth among Libya's three historic regions. The militias also ask for an investigation into allegations of corruption marring oil sales.
