Senior Lieutenant Denis Bakumov, 23, was steering his small, armoured vessel through waters shared by Ukraine and Russia, where tensions are high amid fears of a new Russian invasion.

"Every time we go out at the sea, when we perform some tasks, we constantly bump into them and they constantly follow us and watch us," he told Sky News.

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He said the Russian vessel appeared to be close to a commercial ship on this occasion.

The Ukrainian commander was careful any movements he made would not be seen by the Russian side as a provocation.

The Sea of Azov is a unique flashpoint in the conflict between Moscow and Kyiv.

A vital pocket of water, it is encircled by both countries as well as Crimea, the Ukrainian peninsula that was annexed by Russia eight years ago.

Back in 2004, during less hostile times, an agreement was struck between the two sides to share the waterway, meaning the border of Ukraine and Russia respectively begins at their shores rather than 12 miles out to sea.

However, since Russian President Vladimir Putin seized Crimea and backed an insurgency in eastern Ukraine, his navy and coast guard have also increasingly imposed greater control over the Azov, according to Ukrainian naval commanders.

Sky News was invited onto Senior Lieutenant Bakumov's vessel, called Vyshgorod, for a brief trip out to sea from a small naval base in Berdyansk, a port city in southeast Ukraine.

The fresh-faced officer, dressed in naval camouflage uniform, with a black jumper for extra warmth, said he and a small team of fellow sailors operate the lightly-armoured vessel, which is armed with two weapons systems that can fire grenades and bullets.

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Stepping onto the deck was a daunting prospect in the freezing cold of winter.

Ice and snow had turned the pale grey-coloured metal surface into an ice-rink and there was no railing to catch the clumsy-footed.

Asked whether any sailors had ever fallen overboard, one stern-faced officer said: "No."

Asked whether they had ever lost a journalist, he also said: "No".

Asked whether, were a journalist to plunge into the icy waves, he would dive in and save them, he cracked a smile but again said: "No."

Hopefully a joke.

Warning 'they may use their weapon against us'

The sailors are able to stay out at sea on their boat for up to five days - a tour they typically do more often in the summertime.

During the winter months, trips out are more sporadic.

The team's primary task is to help protect the Ukrainian coast from any Russian attack.

They are also at times told to accompany commercial ships travelling out of the port in case they encounter any problems with Russian patrol ships.

Senior Lieutenant Bakumov said encounters with his Russian counterparts were varied.

"It is not always (aggressive) but (there are) situations when they really start acting aggressively towards us and perform the illegal actions," he said, looking out to sea from his command post.

Asked when that happens, he said: "The dangerous manoeuvring or they warn us that they may use their weapon against us."

He said that happened to him on one occasion when he was accompanying ships transiting close to the Kerch Strait, a particularly sensitive but important area that connects the Sea of Azov to the Black Sea, making it a vital transit point for all trade coming in and out of Ukraine and Russia from that sea route.

Ukraine slowly building back naval power

Ukraine's ability to protect its sea lanes and coastlines became a lot harder after Crimea was taken.

The peninsula had been the home port for the navy.

The majority of Ukraine's naval ships were captured in a devastating blow for a nation that needs the sea for economic trade and security.

But commanders are slowly building back their naval power with the help of international allies - in particular Britain, the United States and Canada.

One plan is to revamp the Berdyansk base, turning it into Ukraine's main naval base on the Sea of Azov within the next couple of years.

A promise by the UK to offer credit to Ukraine to help fund the project means commanders appear confident a shoreline of large stone boulders will soon be transformed into a dock.

"It means that our capacity to resist the enemy is increasing," said another officer, Senior Lieutenant Yaroslav Shevkhenko, 30, gesturing to the area where the modernised base is being built.