ATHENS (Reuters) - Three oil tankers have been damaged by blasts in the last month in separate incidents around the Mediterranean, with the causes unknown, shipping and security sources said on Wednesday.

The incidents are the first involving blast damage to non-military vessels to have taken place around the central Mediterranean in decades.

An explosion caused a one-metre inward breach below the waterline on the hull of Greek-operated crude oil tanker Seajewel at a port in northern Italy on Saturday, one of the shipping sources said. A second blast occurred 20 minutes later on the same vessel without causing further damage.

Italian prosecutors have opened an investigation into the incident that hit Seajewel when it was at anchor in front of Savona-Vado port, Savona's prosecutor told Reuters.

The vessel's operator, Athens-based Thenamaris, did not immediately respond to a Reuters request for comment.

Another vessel operated by Thenamaris, the crude oil tanker Seacharm, was also damaged by a blast off the Turkish Mediterranean port of Ceyhan in late January, two of the sources said.

In a third incident, the Liberia-flagged chemical and products tanker Grace Ferrum was damaged off Libya in February, three sources said, with one adding that the vessel would require a salvage operation.

The vessel was showing its status as not under command on Wednesday off the Libyan coast, LSEG ship tracking data showed.

Its Cyprus-based operator Cymare was not immediately available for comment.

All three vessels had recently called at Russian ports, according to ship tracking data and sources.

Shipping industry officials said on Wednesday there was growing concern over the incidents.

Russian cargo ship Ursa Major sank in the Mediterranean Sea off Spain in late December after an explosion ripped through its engine room and two of its crew were missing, the Russian Foreign Ministry said last month.

(Reporting by Jonathan Saul, Renee Maltezou, Yannis Souliotis, Michele Kambas and Francesca Landini; Editing by Jan Harvey)