In a sign of further deterioration of relations between Moscow and Kiev, Russia is suing Ukraine over the pending delivery of an unspecified number of engines necessary for the production of several Russian warships. The announcement, by state-run United Shipbuilding Corporation, comes amid growing concerns that the crisis in eastern Ukraine has forced the Russian navy to suspend construction of several next-generation warships.

“[United Shipbuilding Corporation] intends to demand from the Ukrainian side in court that it fulfills contracts for turbines that had already been paid for,” the company said, in a statement, cited by the Moscow Times.

The turbines are reportedly designed for the Admiral Grigorovich-class and Admiral Gorshkov-class frigates, the Russian counterparts of the U.S. Navy’s Freedom-class and Independence-class vessels. In 2014, Ukrainian President Petro Poroshenko reportedly suspended defense exports to Russia over Moscow’s annexation of Crimea and the Kremlin’s alleged support for pro-Russian rebels in eastern Ukraine. 

Poroshenko's move cut Russia’s shipbuilding industry off from key supplies of equipment produced in Ukraine, halting several Russian military shipbuilding projects, the Moscow Times reported. Russia plans to produce six Grigorovich-class frigates and eight Gorshkov-class frigates, both of which form the centerpiece of the country’s naval expansion efforts.

According to United Shipbuilding, three Grigorovich-class ships and two Gorshkov-class frigates had received the engines before Ukraine halted their deliveries. Deliveries of the new warships to the Russian navy are reportedly scheduled for early 2020.

“Due to the termination of supply [of gas turbines from Ukraine], we cannot complete the construction of surface vessels for the navy,” Dmitry Rogozin, Russian deputy prime minister, said last week, adding that the frigates whose engines have not yet been paid for, would get new ones developed by Russian aircraft-engine manufacturer Saturn.