Authorities said a container ship flying the Portuguese flag collided with a minesweeper of the Greek fleet off the port of Piraeus early Tuesday morning.
Television footage showed the Greek fleet ship leaning to its side and sinking. A tugboat approached her and was expected to tow her ashore.
A coast guard official said that two were slightly injured, and added that the Greek ship's 27-member crew had been taken to a safe place.
The official added that the container ship Maersk Launceston was not damaged.
He added that it was not immediately clear how the collision occurred, but an investigation would be carried out.