A member of the energy committee in Libya's interim parliament, the General National Congress, said "the oil tanker, bearing the name Morning Glory, anchored today at 4:00 am (0200 GMT) at the port of Al-Sidra."
The armed men, who are seeking autonomy for Libya's east, were trying to load a shipment of petroleum "outside the framework of the state," said the GNC member, who spoke on condition of anonymity.
Acting Oil Minister Omar Shakmak denounced the move as an "act of piracy."
Oil installation guards launched the blockade of key petroleum sites last July, initially over price hikes but later to call for autonomy of the eastern region of Cyrenaica, cradle of the 2011 revolt that toppled Muammer Gaddafi.