"There were clashes in the night and we now are controlling the port," Commander Mustafa bin Dardef said on Tuesday.
The commander is from the Zintan brigade, which is attached to the National Transitional Council (NTC), the new ruling body of Libya.
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The capture of the port, located in eastern Sirte, marks a strategic victory for the anti-Gaddafi forces as they battle for control of Sirte, one of Muammar Gaddafi's last remaining strongholds.
The fight for the Mediterranean city has intensified in the past few days with NATO carrying out raids for the third consecutive day on Monday.
NTC fighters have besieged the city from the east, west and south and were on Tuesday cleaning and oiling their weapons for what they said would be a thrust towards the centre, site of Gaddafi's compound and military bunkers.
"As we move closer to the city centre, it's going to be face-to-face street fighting and we are preparing for it," said one fighter, Ali Zaidi.
On Monday, NTC fighters captured a large weapons cache belonging to Gaddafi forces on the eastern outskirts of Sirte.
