“The Rex, a Tanzanian-flagged tanker registered in Zanzibar, was intercepted” loaded with a suspected illegal cargo of Libyan diesel, navy spokesman General Ayoub Kacem told AFP.
A coastguard official in Tripoli, Colonel Abu Ajila Abdelbarri, said the vessel had been under surveillance for several days.
He said a Maltese trawler was also seized for fishing in Libyan waters without authorisation.
The two vessels were diverted to Tripoli, where their crews were to be questioned.
On Monday, the navy said it had seized a Greek-owned oil tanker near the maritime border with Tunisia and detained its crew on suspicion of fuel smuggling.
Plunged into conflict and political chaos since a 2011 revolution that ousted and killed Libya’s longtime leader Colonel Moamer Gadhafi, oil smuggling to Tunisia, Malta and Tunisia has become a lucrative trade.