(Updates quake details)
LONDON, May 24 (Reuters) - A magnitude 6.4 earthquake struck off the coast of northern Greece on Saturday, some 77 km (48 miles) south-southwest of Alexandroupolis, the U.S. Geological Survey said.
The USGS said the quake's depth was 10 km (six miles). There were no immediate reports of casualties or serious damage.
Residents in Istanbul in neighbouring Turkey felt a small tremor that lasted around 10 seconds.
No further details were immediately available. (Reporting by Karolina Tagaris, Editing by Alison Williams/Mark Heinrich)