The coastguard said several people have been rescued
At least four people, including a nine-year-old child, have died after a speedboat and a tourist vessel collided off the island of Aegina, according to Greece's coastguard.
The bodies of three men and the child were recovered from the water, officials said. One of the men killed in the collision is believed to have been the captain of the tourist boat.
The nationalities of the victims have not yet been released to the public.
According to authorities, four other people were injured in the crash. At least 21 people have been rescued by coastguard vessels and private boats.
It is thought around 20 people were on the tourist boat, but it is unclear how many people were aboard the speedboat, officials said.
One witness told Greece's Mega broadcaster: "We saw over eight or nine people in the sea ... some were swimming, others were unconscious, dead."
Aegina's proximity to Athens makes it a popular holiday island for Greeks and foreign tourists.
The tourist vessel had been transporting people from Aegina to a beach on the nearby deserted island of Moni, a common trip that runs daily.
Referring to the area between the two locations, Aegina mayor Dimitris Mourtzis said: "It's such a small canal, just a few minutes' distance."
Details of how the accident occurred are remain unclear, and the weather in the area was reported to be good.
The Irish Department of Foreign Affairs here says it is aware of the incident and stands ready to provide consular assistance if requested.