Turkey's key Bosporus Strait could be closed for up to three days due to snowstorms.
The Istanbul waterway was shut to all traffic on Thursday as visibility was reduced by bad weather.
Ship agents said at least one north-bound tanker was awaiting its reopening.
The Dardanelles was not affected.
Gale warnings have been issued for part of the Sea of Marmara, Black Sea and Aegean.
