The recent buildup in traffic in the Black Sea eased on Monday, as the number of tankers waiting to pass through Istanbul’s Bosporus on the way to the Mediterranean fell to 13 from 17 a day earlier, a shipping agency said.

In a new measure that has been in force since the start of this month, Türkiye is requiring vessels to provide proof they have insurance in place for all circumstances covering the duration of their transit through its straits or when docking at Turkish ports.

Five tankers were scheduled to go through the Bosporus southbound on Monday, the Tribeca shipping agency said. The number of ships waiting in the Black Sea to pass through the strait had stood at 20 on Friday.

The average waiting time for tankers decreased to 2.8 days from 4.2 days a day earlier, the Tribeca data indicated. The average waiting time peaked at above six days last week.

Türkiye’s maritime authority said it would continue to keep out of its waters oil tankers that lacked appropriate insurance letters.

On Sunday, the authority said four tankers, carrying some 475,000 tons of oil, had provided the necessary insurance letters according to regulations, facilitating their passage through the strait on Monday.

In a statement, the authority also said it removed five oil tankers from the country’s territorial waters via the Dardanelles, further south than the Bosporus, as they could not provide confirmation letters for their insurance.

At the Dardanelles, two tankers were scheduled to pass through southbound on Monday, while seven tankers were waiting to be scheduled, Tribeca said.

The Transport and Infrastructure Ministry’s Directorate General for Maritime Affairs last week said the insurance checks on ships in its waters were a "routine procedure" and stressed it was unacceptable to pressure Türkiye over the measure.

The regulation came into effect before the G-7, the European Union and Australia agreed to bar shipping service providers like insurers from helping export Russian oil unless it is sold at an enforced low price, or cap, to deprive Moscow of wartime revenue.

With that mechanism, Western insurers are required to retain proof that Russian oil covered is sold at or below $60. The industry has a 45-day transition period and a 90-day grace period if the G-7 changes the price cap at a later date.

Officials have said Western insurers started to cancel the insurance and that the majority of international insurers no longer provide coverage for Russian crude.

The Turkish authority said that in the event of an accident involving a vessel in breach of sanctions it was possible the damage would not be covered by an international oil-spill fund.

"(It) is out of the question for us to take the risk that the insurance company will not meet its indemnification responsibility," it said, adding that Türkiye was continuing talks with other countries and insurance companies.

Considering their vital commercial importance as they connect the Black Sea with the rest of the world, the Turkish Straits became a major point of discussion with the eruption of the Russia-Ukraine war on Feb. 24.

The passages are among the world’s most difficult waterways to traverse, as ships must deal with strong currents, sharp turns and varying weather conditions.

The 1936 Montreux Convention guarantees freedom of navigation for merchant vessels passing through Türkiye’s two straits. But it also gives Türkiye the right to regulate security – a provision it is now using to make sure the oil ships are insured against spillage and other accidents.

Millions of barrels of oil per day move south from Russian ports through Türkiye’s Bosporus and Dardanelles into the Mediterranean.

The Bosporus, a 17-mile waterway that connects the Black Sea to the Marmara Sea and eventually to the Mediterranean Sea, is one of the world’s most important choke points for the maritime transit of oil. The 40-mile Dardanelles separately connects the Marmara Sea with the Aegean and Mediterranean.