At least 20 oil tankers queuing off Turkey to cross from Russia's Black Sea ports to the Mediterranean face more delays as operators race to adhere to new Turkish insurance rules added ahead of a G7 price cap on Russian oil, industry sources said.

Turkish maritime authorities issued a notice seen by Reuters last month asking for additional guarantees from insurers that the transit through the Bosphorus would be covered starting from Dec. 2.

The new rule was announced before a $60 per barrel price cap was imposed on Russian seaborne crude this week, Western insurers are required to retain proof that Russian oil covered is sold at or below that price. The industry has a 90-day grace period to comply with the G7 plan.

"Extra coverage from Russian P&I seems to be the way out for tanker operators," the shipping source said, referring to protection and indemnity insurance providers.

"We'll see further delays if owners (or) operators can't provide the required guarantees."

Norway's Skuld, among the top tier of P&I clubs, said such insurers cannot provide the level of detail that has been required.

"The Turkish Government's requirements go well beyond the general information that is contained in a confirmation of entry letter," Skuld said in a note.

Millions of barrels of oil per day move south from Russian ports through Turkey's Bosphorus and Dardanelles straits into the Mediterranean.

The shipping agency GAC said on Tuesday that 13 vessels were waiting to transit the Bosphorus strait southbound, all of them oil tankers and with 10 of them holding Kazakh crude after loading at the Russian port of Novorossiisk.

Only two vessels were scheduled for departure on Tuesday, both coming from Russia's Tuapse.

Tribeca shipping agency said nine oil tankers were waiting to transit the Dardanelles southbound on Tuesday.

GAC also said that average waiting times southbound at the Bosphorus on Tuesday was four days for vessels longer than 200 meters, up from one day in mid-November.

For the Dardanelles strait, average waiting times southbound were also around four days, up from 1-1/2 days in mid-November, Tribeca said.

The only tanker passing the straits after the deadline, Vladimir Tikhonov, was insured by Russia's Ingosstrakh , the source added.