For some months, Russian vessels have grown increasingly aggressive toward Ukrainian ships in the Black Sea and the Sea of Azov. Now Ukraine is marshalling a bigger response, with U.S. help.

On Thursday, Ukrainian officials will take possession of two U.S. Coast Guard Island-class cutters in Baltimore. The transfer comes amid budding maritime hostilities that Ukrainian officials worry may herald a second wave of aggressive Russian military action.

Russia maintains a large naval base in Sevastopol on the Ukrainian peninsula of Crimea, which Moscow illegally seized in 2014. Around August 14, Russia began what many call a blockade of the Kerch Strait, the narrow body that connects the Azov with the Black Sea.

“Experts in Ukraine say this could be the next [Russian] operation from this flank, from the sea, near Mariupol,” Valeriy Chaly, Ukraine’s ambassador to the U.S., said in an August interview at the Ukrainian embassy.

U.S. and Ukrainian officials say Russian military activity has also been increasing in the Azov Sea, which Ukraine and Russia legally share under treaties signed in 2003.

Here’s an Aug. 30 statement from the State Department: “The United States condemns Russia’s harassment of international shipping in the Sea of Azov and the Kerch Strait. Russia has delayed hundreds of commercial vessels since April and in recent weeks has stopped at least 16 commercial ships attempting to reach Ukrainian ports.”

The message from Ukraine has been more urgent. “They are militarizing the Azov, bringing in weapon systems, more troops,” Chaly said. “It’s a disproportionate situation because Russia has more ships. They want to prevail in the Black Sea and Azov not only against Ukranians but internationally...In fact, Russia wants to maintain control and not allow free access of ships.”

It’s not just a matter of seizing marine territory. Moscow’s broader objective is to exacerbate economic hardship in Ukraine, in order to exacerbate civil strife and hasten the collapse of the anti-Russian Poroshenko government.

“We lose money through our ports. Russia is trying to destroy this opportunity for Ukraine,” Chaly said. “It's another tool of … economic pressure to not allow us to have a normal trade relationship. It’s absolutely unacceptable.”

The Island-class cutters, which can carry a 25mm machine gun and several .50-caliber machine guns, are small compared to what the Russians can bring to bear. The Ukrainian government approved the $10 million sale in September.

“We can’t create a fleet like Russia has but we can create so-called Mosquito fleet: small ships but many of them,” said Chaly.

The Ukrainian navy also conducts a exercises called Sea Breeze with the U.S. 6th Fleet, the 18th iteration of which took place in July. It’s a part of Ukraine’s efforts to further develop interoperability with NATO.