Militants in eastern Libya have warned the government against any attacks on a North Korean oil tanker loading crude at a terminal under their control.

On Sunday, Abd-Rabbo al-Barassi, the head of the self-declared rebel movement, warned against harming the tanker, saying such a move would be a "declaration of war," Reuters reported.

The ship, the Morning Glory, started loading at the Es Sider port on Saturday despite earlier threats by the Libyan government to bomb the ship.

Culture Minister Habib al-Amin, who acts as informal government spokesman, said the navy had dispatched boats to stop the 37,000-ton vessel from leaving the port.

"Now the tanker's movements are under complete control and nobody can move it," said al-Amin, adding, "The tanker will stay where it is."

"All efforts are being undertaken to stop and seize the tanker, if necessary by a (military) strike, if it does not follow orders," he added.

Tripoli has made efforts to end a wave of protests at oil ports and fields across the country that have drastically reduced its oil output.

Libya has been witnessing numerous clashes between government forces and rival militia groups, who played a key role in the 2011 popular uprising that toppled former dictator Muammar Gaddafi.

Since August 2013, the rebels have managed to seize three major Libyan ports.

The rebels refuse to lay down arms despite efforts by the central government to impose law and order.

During a conference held in the Italian capital Rome on Thursday, senior officials from over 40 countries said that the ongoing tensions in Libya could slip out of control if a political solution was not found.

"The situation in Libya is very worrying," French Foreign Minister Laurent Fabius told reporters on the sidelines of the second Ministerial Conference on International Support to Libya.