Two ships, the Virat and the Kairos, were rocked by explosions off Turkey’s coast late Friday, according to the Turkish transport ministry, with the Virat struck again early Saturday.

A Ukrainian security source claimed responsibility for the attacks to AFP, saying the ships were covertly transporting Russian oil in contravention of western sanctions over the Kremlin’s invasion of Ukraine.

“We cannot under any circumstances accept these attacks, which threaten the safety of navigation, the environment and lives in our exclusive economic zone,” Erdogan said of the strikes.

“The conflict between Russia and Ukraine has clearly reached a stage where it threatens the safety of navigation in the Black Sea,” he added.

Erdogan’s criticism came as Ukraine faced pressure on both the military and political fronts, and as negotiators stepped up work on US-led efforts to end the conflict.

“We are issuing the necessary warnings to the parties concerned. We are also closely monitoring developments with a view to ending the conflict and stand ready to contribute at every opportunity,” the Turkish leader said.

Both the Kairos and the Virat, which were Gambian-flagged, are under Western sanctions for being part of the “shadow fleet” used by Russia to bypass restrictions on its oil exports.

Turkey has sought to maintain relations with Moscow and Kyiv throughout the war, offering its services as a neutral ground for negotiations.

It also controls the Bosphorus Strait, a key passage to the Black Sea used to transport Ukrainian grain and Russian oil towards the Mediterranean.