Ukraine’s Border Guard Service on Monday accused Russia of effectively blockading the Kerch Strait by subjecting vessels bound for Ukrainian ports to an extremely slow approval process that allows only a few ships to pass per day.

The Kerch Strait connects the Black Sea to the Sea of Azov and was the scene of a confrontation between Ukrainian and Russian military vessels on November 25.

“The Russian Federation does let the vessels pass, but still, the delay is quite significant. According to our information, there are 104 vessels waiting in line in both directions, set to pass the Kerch Strait,” Oleh Slobodian of the State Border Guard Service of Ukraine said on Monday.

“As for the dynamics, the Russian Federation allows access to only a few vessels that are sailing to the Ukrainian ports in the Sea of Azov. At the same time, those sailing to Russian ports navigate through the Kerch Strait freely,” he added.

Slobodian noted the Russian Federation has no legal authority to control passage through the Kerch Strait in the manner he described.

The Ukrainian Defense Ministry warned Russia on Friday that it plans to send military ships through the Kerch Strait to clear up the shipping bottleneck. “Otherwise, Russia will fully occupy the Sea of Azov,” Ukrainian Defense Minister Stepan Poltorak explained.

Russian officials denied they are enforcing a de facto blockade against Ukrainian ports to apply economic pressure against the Ukrainian government, insisting on Sunday their primary concern is the safety of the bridge built across the Kerch Strait to connect Russia with Crimea.

The bridge in question has been denounced internationally as an effort to consolidate Russia’s illegal annexation of Crimea in 2014. Ukrainian intelligence officials have expressed concerns about Russia’s use of the bridge to move missile launchers and other military assets into Crimea. A sizable Russian military convoy was spotted crossing the bridge and entering Crimea on Saturday.

Russia continues to defy calls from around the world to release the Ukrainian ships and sailors it seized on November 25. In fact, a Russian lawyer representing one of the Ukrainian sailors said on Sunday that new charges could soon be brought against the Ukrainians depending on the “political situation and the political will of the Kremlin.” If the charges this lawyer anticipates are brought, the Ukrainian sailors could face up to six years in Russian prison.

Ukrainian President Petro Poroshenko said on Sunday he has contacted the head of the Ukrainian Orthodox Church of the Moscow Patriarchate and asked him to intervene on behalf of the captive sailors.

“I will use every opportunity to get our soldiers back. We will pray and hope that this will happen as soon as possible,” Poroshenko said. He added that Russian President Vladimir Putin has “shunned” his efforts to initiate a dialogue about the Kerch Strait incident.