By Jonny Dymond
BBC News, Istanbul
Turkey is engaged in an international battle to restrict the use of one of the world's busiest waterways - the Bosphorus Straits.
The Bosphorus is wide enough, but its waters can be treacherous
The straits divide European and Asian Istanbul and are the main route for trade from the Black Sea out to the Mediterranean and beyond.
Russia has complained about Turkey's restrictions on the number of oil tankers that can use the channel.
Turkey is used to having international attention focused on the Bosphorus.
For centuries, Russia wanted free access to the channel which links the Black Sea and the Mediterranean.
In World War I, Britain led a disastrous attempt to take the straits and open up a new front in the deadlocked conflict. Now the politics of international trade - in particular, oil - has led to fresh complaints.
Russia insists that it has the right to use the international waterway and that Turkey's unilateral limiting of the number of tankers going through the straits is illegal.
Turkey says the rules governing the passage of ships down the Bosphorus were designed for a different age.
The Turkish Maritime Authorities are concerned that large tankers carrying oil, gas or chemicals pose a serious danger to the 12m or so residents of Istanbul.
The authorities say 9,000 ships carrying explosive materials went through the straits last year.
The tonnage of explosive material passing through has more than doubled in seven years. The Bosphorus looks wide enough. But it is treacherous.
Ships can be swung around and sent into the city's coast if captains are not alert to the twists and turns of the straits and the currents below the surface.
Turkey has invested millions of dollars in a radar monitoring system.
It insists that only one tanker travels the straits at any one time and that no ships go down in bad weather or at night time.
Some observers believe it to be a ploy to force investment in pipelines across Turkish territory.
But the authorities insist that the safety of the city and its residents is their key concern.