The number of tankers waiting to pass through Istanbul's Bosphorus Strait on the way to the Mediterranean fell to eight on Tuesday from 13 a day earlier, the Tribeca shipping agency said, in a further easing of the recent build-up in traffic.

A Turkish measure in force since the start of the month has caused shipping delays. It requires vessels to provide proof of insurance for the duration of transit through the Bosphorus Strait or when calling at Turkish ports.

Five tankers were set to go southbound through the Bosphorus on Tuesday, Tribeca said. On Friday, there had been 20 ships waiting in the Black Sea to pass through the strait.

At the Dardanelles, further south than the Bosphorus, eight tankers were set to pass through southbound on Tuesday, while six tankers were waiting to be scheduled, Tribeca added.

The average waiting time at the Bosphorus for southbound tankers fell to 2.9 days to 3.4 days from 3.8 days to 4.3 days on Monday, Tribeca said. Average waiting time peaked at above 6 days last week.