Merchant ships on Monday continued to be congested in lanes around the Black Sea, as ports grappled with the challenge of resolving backlogs amid growing concerns among insurers and shipping companies after a Russian warship fired warning shots at a cargo vessel.

Russia said its Vasily Bykov patrol ship on Sunday fired on the Palau-flagged Şükrü Okan vessel after its captain failed to respond to a request to halt for an inspection. After an inspection, the vessel continued its journey toward the Ukrainian port of Izmail along the Danube River, Russia said. Kyiv on Monday condemned what it called "provocative" Russian actions and called for decisive countermeasures by the international community.

Insurance industry sources said rates for additional war risk premiums remained stable on Monday, although there was a possibility of a rise if a ship was damaged or sunk.

The cost of a Black Sea war risk premium, which is typically renewed every seven days and is in addition to annual insurance expenses, was estimated at tens of thousands of dollars per ship for the voyage.

At least 30 ships had dropped anchor around Musura Bay in the Black Sea, which leads into a channel that links up with Izmail further along the waterway, tracking data from analytics company MarineTraffic showed on Monday.

There were at least 20 ships anchored leading up to Izmail. In addition, there were at least 35 commercial ships waiting close to the Romanian port of Constanta, 15 more than last week, the MarineTraffic data showed. Many of the vessels had reported their destination as Romanian ports.

Romania on Monday said that it aimed to double the monthly transit capacity of Ukrainian grain to Constanta to 4 million tons in the coming months. Sunday's incident cast a pall over plans announced by Ukraine last week for a "humanitarian corridor" in the Black Sea to release cargo ships trapped in Ukraine's ports since the outbreak of war.

There are an estimated 60 vessels still stuck inside Ukrainian ports including Odesa, one of three terminals that were part of Türkiye- and the United Nations-brokered Black Sea Grain Initiative (BSGI) that Moscow exited.

Moscow says it will return to the grain deal only if it receives better terms for its own exports of food and fertilizer. Turkish President Recep Tayyip Erdoğan, the biggest sponsor of the grain deal alongside the U.N., says he hopes to persuade Russian President Vladimir Putin to rejoin it at talks this month.

The Joseph Schulte was among the vessels that remained stuck in Odesa.

"We continue to do everything in our power to enable the vessel to move, despite permits and the many variables involved," a spokesperson with Schulte Group, the parent of the vessel's German-based manager BSM, told Reuters on Monday. "The situation remains complex."