Salvini said weather conditions were not good and it was too early to tell if the ship was being directed toward Malta or Italy's Lampedusa island.
Over the years, Europe has added new functions to the mission, which also trains the Libyan coastguard and controls the enforcement of a United Nations arms embargo on Libya, and illegal oil trafficking.
Maltese officials were monitoring the situation and were prepared for any eventuality, the spokesperson added.
"Immigration is managed by criminals and should be blocked by any legal means necessary", Salvini was quoted as saying by the Italian news agency ANSA.
"They are pirates", Salvini said in Italian in a Facebook post on Wednesday.
The tanker was about 30 nautical miles (55.5km) from Malta when the Armed Forces of Malta (AFM) established communications with the captain. Four migrants were taken off the tanker in handcuffs. The bloc's Operation Sophia has worked to reduce migrant-smuggling, train the Libyan coast guard and enforce an arms embargo on Libya. Several police vans were parked on shore waiting to take the migrants into custody.
"We have been told the migrants on board the vessel were fearful of being returned to Libyan detention centres where they could face violations at the hands of people smugglers", Abdelwahed said.
Matteo Salvini, the country's hardline interior minister, has condemned the migrants as "pirates".
The European Union has been training the Libyan coast guard in the hope that it will prevent migrants from entering global waters, where they have routinely been rescued, either by commercial ships or those run by humanitarian groups filling in the void after member countries significantly scaled down an EU operation in the Mediterranean. If they pick up a large number of people from a floundering vessel and even a few of them have weapons, it probably wouldn't be that hard of a trick to pull off. After returning control of the ship to its captain, the AFM are now escorting the ship to Boiler Wharf and is expected to dock imminently.
A group of migrants has "hijacked" a merchant ship that appears to be heading towards Malta, the Armed Forces of Malta said on Wednesday.
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