An Italian helicopter was flying to intercept a cargo ship believed to be carrying hundreds of migrants after it left Greek waters and headed towards Italy, according to a statement from the air force.
The Moldovan-flagged ship, Blue Sky M, was spotted close to the coast of Corfu earlier in the day after authorities got an alarm call about a potential incident on board.
Greek state television said the alarm had been raised because armed men were on board, but there was no confirmation from the defence or shipping ministries.
Greek naval and coastguard vessels as well as a military helicopter were sent to the scene, but the ship was allowed to continue into international waters after the captain assured officials the situation was under control.
"He said there was no problem and he would continue on course towards Italy," said a shipping ministry official, who declined to be quoted by name.
Greece's SKAI television said as many as 700 people may be on board, but other reports put the total at 400.
The Italian airforce said a helicopter had taken off from its base in Gioia del Colle in southern Italy and was carrying coast guard officials.
Italy and Greece have seen more than 100,000 immigrants arrive on their shores from the Middle East and North Africa, often travelling in small, unsafe boats. Thousands have died in the attempt.
The civil war in Syria and a state of near-anarchy in Libya have worsened the crisis.
(Reporting by Angeliki Koutantou, Lefteris Papadimas; Writing by James Mackenzie; Editing by Andrew Heavens, Larry King)