Mariupol, Ukraine – Setting out on flat waters in the space between the Gray Sea and the sky, the Ukrainian Coast Guard patrols the Sea of ​​Azov in search of unregistered vessels, contraband and, since 2014, Russian warships.

The world’s shallowest sea sits precariously between Ukraine, Russia and annexed Crimea. With winters in the region so cold that the sea freezes for the first few months of the year, the thaw that arrived this week brought a new struggle – the threat of coastal invasion.

“We used to watch fishing boats and nothing else,” says Ihor Chertov, deputy head of the coastguard unit in the port city of Mariupol in eastern Ukraine.

“Now it’s the muscles of the aggressor state, Russia, always trying to push the limits and get closer to shore than they are legally allowed to.”

With all eyes on 10 days of military exercises in Ukraine’s northern neighbor Belarus and fears of an attempted takeover of the capital Kiev, Russia has moved closer by water, displacing six warships and one submarine from the Mediterranean to the Black Sea.

A closure of the southern part of the Sea of ​​Azov for large-scale naval exercises next week was announced and then lifted, deepening confusion over Russia’s intentions as the West continues diplomatic efforts to ease tensions. .

Raptor boats and attack helicopters were seen moving towards Russian coastal positions, as Moscow ships patrolled the water. The situation is particularly claustrophobic in Mariupol – its port brings vital income to Ukraine through exports, while the city sits between the Russian border, territory controlled by pro-Moscow separatists, and Crimea.

The peninsula was seized by Russian soldiers in 2014 and has remained under their control ever since.

“The Russian ships are 50-60 meters (164-197 feet) long and you have to understand that they have not just arrived – they are still stationed here. Always on the lookout, since 2014”, says Chertov, 47, facing a calm sea despite his environment.

He served in the Marine Guards for 27 years and knows only too well what is at stake if the current tensions escalate into conflict – he previously served in Crimea, leaving just before Russian forces arrived, and his parents there. still live.

Ihor Chertov, 47, deputy head of the base unit in the Sea of ​​Azov [Emre Caylak/Al Jazeera]

Life under Russian President Vladamir Putin’s long arm is subject to many restrictions, and Chertov is too concerned about their safety to openly criticize Moscow.

In December, he was at the helm of the ship when a Russian vessel entered the commercial port, already under threat due to Russian restrictions on freedom of navigation. Russia and Ukraine have equal rights to use the Sea of ​​Azov, but Moscow claims control of the Kerch Strait – the only narrow passage.

“We had to approach the boat to a proximity that was not comfortable for us physically and chase them away,” he says. “We can expect anything from Russia.”

The Russian Navy has been harassing Ukrainian ships for nearly a decade, and in 2018 three Ukrainian military vessels were hijacked by Russian border guards and 24 sailors were taken hostage.

The Mariupol Coast Guard said it is now preparing by undertaking additional training, particularly on how to deploy drones and deal with enemy drones.

Captain Oleksandr Surkov, 32, of the coastguard boat Onyx, is from Mariupol and has served in the coastguard since 2007. He says the Russian navy often tries to get them to take the first step in trying to stir up trouble.

“They always try to provoke the Ukrainian side. They enter our waters and wait for our third warning to leave, always trying to push the limits by getting too close,” he says.

Navy ship Onix in the Sea of ​​Azov [Emre Caylak/Al Jazeera]

On Thursday, US President Joe Biden urged Americans in Ukraine to leave the country immediately, warning that “things could get crazy fast” in the region. British Prime Minister Boris Johnson said the crisis had entered “the most dangerous moment” during a visit to NATO headquarters.

Both have been accused of exaggerating the current threat posed by Russia, while Ukrainian President Volodymyr Zelenskyy downplays it.

However, with tensions so high, any misstep could turn a crisis into a full-fledged conflict, and if a Russian ship comes within 700 meters (2,297 ft), or 3 km (1.86 miles ) for larger ships, the Coast Guard of the Sea of ​​Azov is supposed to shoot.

For Surkov, who recalls the battles fought in Mariupol in 2014 after Russian-backed separatists briefly took control of the city, Russia should stop trying to provoke a reaction and leave Ukraine in peace.

“Stick to your own territory, OK, there’s no need to come here,” he says.