Ukraine is already defending itself against Russian attacks on multiple fronts and on Thursday, with Russian warships clearly visible off the coast of Odessa, it was stepping up its preparations against a potential amphibious landing.

According to local residents, airstrikes against the key southern city have become more frequent over the last day.

More disturbingly, there are now concrete U.S. warnings of a landing of Russian forces from the sea as early as Thursday.

According to The Guardian, the convoy appears to include several 4,080-ton Ropucha-class large landing ships, specially designed for beach landings, along with supporting vessels. Each Ropucha ship can carry a cargo of up to 450 tons or as many as ten battle tanks. Russia has previously sent such ships to Syria, carrying both military supplies and troops.

Providing further evidence that Russia made the decision to invade Ukraine well in advance, TheGuardian also notes that several Ropucha-class ships passed through the Straits two weeks before war broke out, supposedly in order to conduct drills with another Russian landing ship in the Black Sea.

Ukraine's armed forces are now engaged in laying mines on Odessa's beaches and preparing additional defenses.

Odessa is Ukraine's third-largest city, after Kyiv and Kharkiv, and also a popular tourist destination. In and around the city are important oil and chemical processing facilities, and Odessa also has two large ports, the Port of Odessa and Port Yuzhne.

The coastal city of Kherson has already fallen to the Russians, and now Mariupol has been surrounded by Russian troops, according to Ukrainian Interior Ministry advisor Anton Herashchenko. Losing Odessa, a city of over a million people, would deal a devastating blow to Ukraine's economy as well as to its morale.

According to a study made in 2015, Odessa is 68% ethnic Ukrainian and 25% ethnic Russian.