Both vessels are reported to have sustained damage above the waterline during the incident.
A bunkering operation was halted just prior to a product tanker alliding with a gas tanker on Sunday at the port of Ceuta, Spain, local media reports.
The 6,100 DWT FT Foce, which is operated by Furtrans Denizcilik (Furtrans), hit the stern of the moored 4,512 DWT Unigas liqufied petroleum gas (LPG) carrier Happy Eagle.
Happy Eagle is said to have been in the midst of the bunkering operation when a crewman noticed FT Foce's approach.
The LPG's chief officer is said to have alerted personnel ashore to halt the bunkering operation before contact between the two vessels was made.
Both vessels are reported to have sustained damage above the waterline as a result of the incident.
No pollution or injuries have been reported.