MADRID, March 19 (Xinhua) -- Italian authorities in Sicily have detained the Spanish rescue vessel Open Arms for promoting "illegal immigration and criminal association," the Spanish charity which runs the ship said Monday.

Proactiva Open Arms said the ship was confiscated in the port of Pozzallo after rescuing 218 migrants 70 nautical miles off the Libyan coast.

According to the Spanish charity, the Italian public prosecutor accused those on board of "violating the law and international agreements by not handing the migrants to Libyan authorities," even though the ship was allegedly well out of Libyan territorial waters when the rescue took place.

Founder of Proactive Open Arms, Oscar Camps, told Catalunya Radio the ship was under "preventative detention" in Pozzallo and confirmed that Italian police had taken the ship's captain, Marc Reig, and the head of Proactive Open Arms on the boat to a local police station on Sunday.

Camps explained Libyan coast guards had threatened the vessel and demanded the migrants be returned to them, but commented on Twitter that doing this would have been the "equivalent of a summary deportation."

Camps said the Italian authorities had opened an investigation into the "promotion of illegal migration," and warned that the crew faced possible prison sentences if eventually found guilty. Camp said this was to "block the intervention of humanitarian groups."

According to the International Organization of Migration, 3,116 migrants died attempting to cross the Mediterranean Sea in 2017. A further 318 died in the early weeks of 2018.