MARIUPOL, Ukraine -- One recent summer morning, four Ukrainian Sea Guards peered out from a speeding patrol boat nicknamed Little Boy. Bubbly pop hits from the 1990s blared over the hum of the engine propelling them through the choppy swells of the Sea of Azov. The airy tunes contrast sharply with the rising tension in this sea, stoked, Ukraine says, by its powerful and ever-encroaching foe lurking in the waters: Russia. Around six nautical miles from the port of Mariupol, a symbol of that new tension came into view: the Ukrainian armed cutter-class ship Lyubomir, bobbing on the waves. From its top deck, one guard trained his eyes on the horizon as another stood rigid behind a heavy machine gun on the lower deck. No Russian naval or border guard vessels were visible that morning. But both have been increased in recent months, so they were out there somewhere. "We have to always be ready," said Artem Poliakov, a Sea Guard and the squadron's spokesman. A New Flash Point Back ashore, in and around Mariupol, a crucial industrial port city with a population of around 500,000 that sits roughly 800 kilometers southeast of Kyiv, it has been tense since late in the spring of 2014. Russia-backed separatists briefly controlled it then before the Ukrainian military and its volunteer battalions dislodged them. Today, fighting continues to rage just 24 kilometers to the east, in the once-quiet seaside town of Shyrokyne.

But now tensions are spilling into the sea that washes the 300-kilometer shoreline Ukraine still controls after the annexation of Crimea. The Sea of Azov, a rich fishing ground in Soviet times that has been of great strategic importance to independent Ukraine, has emerged after months of growing friction as the latest flash point in the four-year conflict between Moscow and Kyiv. Bridge Construction, Arrests, Navy Deployment Russia launched the opening salvo here in 2016, with its $3.7 billion Crimean Bridge project to link that occupied Ukrainian peninsula with southern Russia across the Kerch Strait -- the gate to the Azov. The move led to stern condemnation and sanctions from Kyiv and Western governments. The European Union in late July added six more Russian companies involved in the bridge project to its sanctions list. In March, with the bridge nearing completion, Ukrainian authorities detained a Ukrainian-registered Crimean fishing vessel for illegally sailing under the Russian flag and arrested its captain and crew in the Sea of Azov -- a move one Russian official likened to that of "Somali pirates." Since then, Russia has stepped up confrontation in and around the Azov in a big way. In early May, Russian border guards subordinate to the country's Federal Security Service (FSB) reportedly arrested a Ukrainian fishing vessel and its crew in the Black Sea, ostensibly for illegally fishing in what it said was Russia's exclusive economic zone.

In mid-May, following the Crimean Bridge's completion, Russia reportedly moved naval vessels including warships from its Caspian Flotilla to the Sea of Azov, citing a need for stepped-up security around the new structure. Since then, Russia's FSB has detained more than 148 Ukrainian and foreign merchant ships -- many more than once -- and interrogated their crew members, Ukrainian officials, port authorities, local shipping companies, and experts told RFE/RL. Such activities have not led to any shots being fired. But the appearance of armed Russian ships and the increased activity of its FSB alone mark a dangerous escalation and highlight an imbalance of power at sea that has put Mariupol on edge again. "We have Russian vessels floating nearby, ready to attack from the sea. And each provocation may turn this situation into a war," Galina Odnorog, a co-founder of the Mariupol Social Movement, a local nongovernmental organization focused on issues related to the Sea of Azov, told RFE/RL. "We are in a tinderbox." And there might be little, if anything, Ukraine can do about it. Who Controls The Sea? Poliakov said that, while Russia's actions are "provocative," because of a controversial 2003 agreement on cooperation and shared use of the Sea of Azov and Kerch Strait, "everything Russia is doing here is technically legal." Signed by Russian President Vladimir Putin and Ukraine's then-President Leonid Kuchma in 2003, the agreement makes the Sea of Azov jointly controlled territory, allowing both countries to use it freely. "Ukraine and the Russian Federation, two historically brotherly nations, define the Sea of Azov and Kerch Strait as economically important for both countries," the agreement reads. And despite the current conflict between Kyiv and Moscow, the agreement is still in place. There have been calls in Ukraine to rip it up. But Oleh Slobodyan, chief spokesman for the Border Guard Service of Ukraine, ruled out making any change to the agreement, saying such a move would only work if Russia also intended to implement the changes. "We can make such a decision unilaterally, but, in my opinion, this will not have any effect" on Russia's behavior, he said. 'Sea Blockade' That deal, said observers and officials, has allowed Russia to impose an effective "sea blockade" in the Azov, a measure meant to achieve several goals. "The sea blockade that Russia is conducting is aimed, first, at delivering economic losses to Ukraine, and, secondly, at creating greater tensions in cities like Mariupol and other small cities and villages where people are making their living in sea trade and fishing," said Oleksiy Melnyk, a Ukrainian political and security analyst at the Kyiv-based Ruzumkov Center, a nongovernmental public-policy think tank. Mariupol and nearby Berdyansk are home to Ukraine's two major steel- and grain-exporting ports on the Sea of Azov and hundreds of private fishing companies that employ more than 20,000 people. The ports in both cities have experienced steep cuts in cargo handling since construction began on the Crimean Bridge, while the waters where fishermen can freely move has been reduced due to fears of being arrested by Russia's FSB. Because of the low height of the Crimean Bridge, 144 ships that rise taller than its 33-meter clearance can't reach the Port of Mariupol, resulting in a 30 percent loss in revenues for the port, according to official statistics provided to RFE/RL. To avoid layoffs, the port has shortened the work week to four days for most of its 3,000 employees. When RFE/RL visited the port in late July, only one merchant ship was present, and the dozen or so workers on duty were making repairs to a large crane and a pipe running from the dock.

Ukraine's minister of infrastructure, Volodymyr Omelyan, reportedly said during a program on the 112 Channel this month that there has been a 10 percent decrease in calls to the ports of Mariupol and nearby Berdyansk. But he also tried to downplay the troubles, calling the situation in and around the Sea of Azov "stable." His comments came after Ukraine's deputy minister for European integration, Olena Zerkal, downplayed the situation in early July, saying it had been "artificially created by the media." 'Lack Of Reaction From Kyiv' Odnorog said that is not the case, arguing that Kyiv has not paid enough attention to the escalating "crisis" in the sea. "There is also a problem with the reaction of our authorities -- namely, a lack of reaction," Odnorog said. Others in Mariupol also shake their heads at official remarks from Kyiv. In explaining the losses his company has suffered in recent weeks, Anton Shapran, director at the Mariupol-based Maritime Logistics, a management company and service provider for 70 ships in the Sea of Azov, insisted the "crisis" was real.

He said that owners of "several" vessels had complained to him about having been detained for longer than 24 hours -- a delay that can cost between $3,000 and $13,000. Some of the ships have been stopped for more than three days, according to Andriy Klymenko, editor in chief of Black Sea News, an online information site devoted to covering developments in the Black and Azov seas. And the delays are growing. As they do, several shipping companies are thinking twice about trips to Ukrainian ports in the Sea of Azov, according to Odnorog and officials at the Port of Mariupol, who asked for anonymity because they were not authorized to provide official comments to media. "Many vessels already refuse to enter the Sea of Azov," Odnorog said, adding that the average delay time for vessels detained and searched over the past week of July had climbed to 54 hours. Beginning Of Sea Annexation? And there are other likely reasons for Russia's sea blockade. Balazs Jarabik, a nonresident scholar at the Carnegie Endowment for International Peace, speculated that Moscow might be trying to pile pressure on Ukraine to force it to "ease" a ban on supplying water to occupied Crimea, which appears to be drying up at an alarming rate.

Ukrainian state water agencies stopped flows through a major canal in 2014, citing millions in outstanding debts from the peninsula's Russian-imposed government. Still others say Moscow's ultimate goal may be nothing short of taking complete control of the waters surrounding annexed Crimea and all along the Ukrainian coastline. "If on [the Ukrainian] side there will not be increased patrolling, demonstrating the intentions of ousting Russian boats, [and] a sharp appeal to the Russian Federation and the international community, then this may become the beginning of the annexation of the Sea of Azov," wrote the Black Sea News's Klymenko. Protecting The Shore, But Not The Sea? Ukraine is trying a raft of tactics to respond to Russia's maneuvering. It is negotiating with European and U.S. partners to punish Russia's Black Sea ports over Moscow's moves in the Sea of Azov, according to Omelyan, the infrastructure minister. But it is unclear how persuasive Kyiv has been when so much of the West is grappling with its own problems. Activists are also trying their hand at attracting international help. Maria Podybailo, Odnorog's fellow co-founder at Mariupol Social Movement, said they had sent a letter to the Organization for Security and Cooperation in Europe's Special Monitoring Mission (OSCE SMM) -- a group of international observers tracking the fighting on the battlefields of eastern Ukraine -- asking them to expand their mandate to the sea "to monitor and record violations." It is unclear whether the OSCE SMM is considering the proposal. The OSCE SMM did not respond to RFE/RL requests for comment. Ukraine is taking measured military actions along the Sea of Azov shore after President Petro Poroshenko warned on July 16 that Russia was building up forces and weapons in the Black and Azov seas. He said he did not exclude the possibility that the Kremlin might prepare for an all-out attack on Mariupol. The next day, Poroshenko ordered top military officers to take measures in connection with the ship detentions and inspections, including possibly providing escorts through the sea. "We will not tolerate the illegal seizure of Ukrainian and foreign ships that are moving toward Ukrainian ports, including Mariupol," the president said. In a show of force, the Ukrainian military conducted two-day helicopter shooting drills over the Sea of Azov in late July. "The Joint Forces are paying considerable attention to the defense of the Azov coast to prevent the landing of enemy amphibious assault troops," said Commander of the Joint Forces of Ukraine Serhiy Nayev. "All units that are involved in the defense of the sea coast shall conduct regular training to boost the readiness to repel an attack." But there seems to have been little done to boost defenses on the sea itself.