A merchant ship under U.S. sanctions passed Turkey’s Bosphorus Strait on its way from Syria to Russia late last month. European intelligence officials who tracked the Sparta II say it carried military vehicles to bolster President Vladimir Putin’s war in Ukraine.

The ship’s journey to the Black Sea port of Novorossiysk underlines the Kremlin’s efforts to tap resources for the invasion that’s now in its sixth month, as supply lines strain under the pressure of Europe’s largest military campaign since World War II.

While Ukraine has received billions of dollars of weapons from the U.S. and Europe to help defend itself, Russia must rely on its own resources to support front-line forces amid reports of extensive losses. Tens of thousands of Russian troops have been killed or wounded and thousands of armored vehicles destroyed, according to U.S. estimates.