“The guns have been firing for four years already,” Ukrainian military volunteer Galina Odnorog says in between shouting instructions to her team, and talking into the mobile phones seemingly pinned to either ear.

Mariupol - a Ukrainian port city between mainland Russia and Crimea - is “used” to being on high alert, she said: “We know Russia can attack us at any time.”

War has, indeed, never been far away from this town on the Azov Sea. The first lines of trenches, which appear just a few miles east of the city boundaries, speak of the current fighting. The damage and craters in buildings inside the town speak of the past fighting.

But last Sunday’s skirmishes in the Kerch Strait – which saw Russian forces block, ram and then fire on a Ukrainian flotilla trying to access the shared Azov Sea and Mariupol – have put the threats facing the city back in focus.

For a long while, locals assumed the main danger came from a direct invasion – by land and by sea. In the event, the crisis has come from much less tangible threat – a de facto blockade of merchant ships headed for the port.

Friday was the sixth day that Russian border guards held boats at the Kerch Strait, the narrow channel that allows entry to the Azov Sea from the Black Sea. As of Friday morning, 20 boats bound for Mariupol and Berdyansk were believed to be waiting. Unconfirmed reports suggest that only boats headed to Russian ports in the Azov Sea have been granted permission to pass the Kerch Strait.

So far, the blockade has had no notable impact on work in Mariupol’s 18-dock port. The Independent understands that on Friday, the port was working with near-normal volumes, loading the ships that were already in dock.

Protests break out after Russia seize Ukraine warships in 2018 Show all 32 1 /32 Protests break out after Russia seize Ukraine warships in 2018 Protests break out after Russia seize Ukraine warships in 2018 Activists of opposition parties burn flares during a rally demanding to break an agreement with Russia on the use of the Azov Sea and the Kerch Strait, in front of the parliament building in Kiev Reuters Protests break out after Russia seize Ukraine warships in 2018 Volunteers with the right-wing paramilitary Azov National Corps light flares during a rally on the snowy streets in front of the Ukrainian parliament in Kiev after Russia seized two of their armored artillery vessels and a tug boat in the Black Sea AP Protests break out after Russia seize Ukraine warships in 2018 Ukrainian Nationalists demand to break the diplomatic relations with Russia and nationalization of Russian property in Ukraine EPA Protests break out after Russia seize Ukraine warships in 2018 Ukrainian activists burn flares during their rally in front of Russian Consulate in Kharkiv EPA Protests break out after Russia seize Ukraine warships in 2018 Activists of opposition parties burn flares during a rally demanding to break an agreement with Russia on the use of the Azov Sea and the Kerch Strait, in front of the parliament building in Kiev, Ukraine November 26, 2018. REUTERS/Gleb Garanich GLEB GARANICH Reuters Protests break out after Russia seize Ukraine warships in 2018 Seized Ukrainian ships, small armoured artillery ships and a tug boat, are seen anchored in a port of Kerch Reuters Protests break out after Russia seize Ukraine warships in 2018 Volunteers with the right-wing paramilitary Azov National Corps brandish flares during a rally on the snowy streets in front of the Ukrainian parliament in Kiev, Ukraine, Monday, Nov. 26, 2018. Some hundreds of protesters from far-right party National Corps brandished yellow-and-blue flags with the Ukrainian national trident symbol, and a banner reading 'Don't back down!" . (AP Photo/Efrem Lukatsky) Efrem Lukatsky AP Protests break out after Russia seize Ukraine warships in 2018 The President of Ukraine, Petro Poroshenko, announced, on 25 November, at a meeting of the National Security and Defense Council that they will introduce martial law in Ukraine for a period of 60 days and appeal to the Parliament to consider the move Reuters Protests break out after Russia seize Ukraine warships in 2018 Russia seized two small-sized 'Berdiansk' and 'Nikopol' armored artillery boats. The 'Yany Kapu' tugboat has forcibly been stopped Reuters Protests break out after Russia seize Ukraine warships in 2018 Children from an orphanage volunteer to help the city defenders strengthen trenches on Ukraine's Army positions near the village of Rybatske 25 km from Mariupol AP Protests break out after Russia seize Ukraine warships in 2018 Volunteers with the right-wing paramilitary Azov National Corps light flares during a rally on the snowy streets in front of the Ukrainian parliament in Kiev, Ukraine, Monday, Nov. 26, 2018. Some hundreds of protesters from far-right party National Corps brandished yellow-and-blue flags with the Ukrainian national trident symbol, and a banner reading 'Don't back down!" (AP Photo/Efrem Lukatsky) Efrem Lukatsky AP Protests break out after Russia seize Ukraine warships in 2018 Policemen guard at Russian consulate during a protest action in the Black Sea Ukrainian city of Odessa AFP/Getty Protests break out after Russia seize Ukraine warships in 2018 epa07190805 Ukrainian Nationalists burn flares and smoke grenades during their rally in front of Parliament building in Kiev, Ukraine, 26 November 2018 as they demand to break the diplomatic relations with Russia and nationalization of Russian property in Ukraine. The President of Ukraine Petro Poroshenko at a meeting of the National Security and Defense Council on 25 November 2018 announced the decision to introduce martial law in Ukraine and to appeal to the Parliament to consider the move at an extraordinary meeting. It is about the introduction of martial law for a period of 60 days. The Ukrainian Parliament will convene for an extraordinary meeting at evening 26 November 2018. Russia has seized three Ukrainian vessels amid their leaving the Kerch Strait on 25 November 2018. The two small-sized 'Berdiansk' and 'Nikopol' armored artillery boats have come under enemy fire and are now dead in the water. The 'Yany Kapu' tugboat has forcibly been stopped. The vessels have been captured by special forces of the Russian Federation, the press service of Ukraine's Navy said on Facebook on Sunday evening. EPA/SERGEY DOLZHENKO EPA Protests break out after Russia seize Ukraine warships in 2018 Activists of Ukrainian far-right groups shout slogans during their rally in front of the Ukrainian parliament in Kiev on November 26, 2018, as they demand to set martial law in the country and to cut diplomatic relations with Russia. - Ukraine's military was on high alert and parliament was to vote on November 26 on a request from President to impose martial law for 60 days after Russian navy seized three Ukrainian ships off the coast of Crimea. (Photo by Sergei SUPINSKY / AFP)SERGEI SUPINSKY/AFP/Getty Images AFP/Getty Protests break out after Russia seize Ukraine warships in 2018 With relations still raw after Russia’s annexation of Crimea and its backing for a pro-Moscow insurgency in eastern Ukraine, the capture of three Ukrainian naval vessels risks pushing the two countries towards a wider conflict. Graphic explains how the naval incident unfolded on a map Graphic News Protests break out after Russia seize Ukraine warships in 2018 Activists of far-right parties burn flares in front of the parliament building in Kiev during a rally to support the Ukrainian navy Reuters Protests break out after Russia seize Ukraine warships in 2018 A Ukraine army APC moves toward on position at an undisclosed location in eastern Ukraine AP Protests break out after Russia seize Ukraine warships in 2018 Ukrainian Nationalists sign a banner with a slogan reading like 'Not retreat and not surrendering!' EPA Protests break out after Russia seize Ukraine warships in 2018 Activists of Ukrainian far-right groups hold flares during their rally in front of the Ukrainian parliament in Kiev on November 26, 2018, as they demand to set martial law in the country and to cut diplomatic relations with Russia. - Ukraine's military was on high alert and parliament was to vote on November 26 on a request from President to impose martial law for 60 days after Russian navy seized three Ukrainian ships off the coast of Crimea. (Photo by Sergei SUPINSKY / AFP)SERGEI SUPINSKY/AFP/Getty Images SERGEI SUPINSKY AFP/Getty Protests break out after Russia seize Ukraine warships in 2018 Activists glue Ukrainian flag on the fence of Russian consulate AFP/Getty Protests break out after Russia seize Ukraine warships in 2018 A volunteer with the right-wing paramilitary Azov National Corps during a rally on the snowy streets in front of the Ukrainian parliament in Kiev AP Protests break out after Russia seize Ukraine warships in 2018 Volunteers with the right-wing paramilitary Azov National Corps hold flares during a rally on the snowy streets in front of the Ukrainian parliament in Kiev, Ukraine, Monday, Nov. 26, 2018. Some hundreds of protesters from far-right party National Corps brandished yellow-and-blue flags with the Ukrainian national trident symbol, and a banner reading 'Don't back down!" (AP Photo/Efrem Lukatsky) Efrem Lukatsky AP Protests break out after Russia seize Ukraine warships in 2018 epa07190891 Ukrainian Nationalists attend their rally in front of Parliament building in Kiev, Ukraine, 26 November 2018 as they demand to break the diplomatic relations with Russia and nationalization of Russian property in Ukraine. The President of Ukraine Petro Poroshenko at a meeting of the National Security and Defense Council on 25 November 2018 announced the decision to introduce martial law in Ukraine and to appeal to the Parliament to consider the move at an extraordinary meeting. It is about the introduction of martial law for a period of 60 days. The Ukrainian Parliament will convene for an extraordinary meeting at evening 26 November 2018. Russia has seized three Ukrainian vessels amid their leaving the Kerch Strait on 25 November 2018. The two small-sized 'Berdiansk' and 'Nikopol' armored artillery boats have come under enemy fire and are now dead in the water. The 'Yany Kapu' tugboat has forcibly been stopped. The vessels have been captured by special forces of the Russian Federation, the press service of Ukraine's Navy said on Facebook on Sunday evening. EPA/SERGEY DOLZHENKO SERGEY DOLZHENKO EPA Protests break out after Russia seize Ukraine warships in 2018 An activist piles up tires during a protest against the seizure by Russian special forces of three of the Ukrainian navy ships, which Russia blocked from passing through the Kerch Strait into the Sea of Azov in the Black Sea, in front of the Russian embassy in Kiev, Ukraine November 25, 2018. REUTERS/Gleb Garanich Reuters Protests break out after Russia seize Ukraine warships in 2018 A National Guard serviceman extinguishes a torch thrown by a protester during a rally against the seizure by Russian special forces of three of the Ukrainian navy ships Reuters Protests break out after Russia seize Ukraine warships in 2018 A protester throws a smoke grenade during a rally in front of the embassy of Russia in Kiev AP Protests break out after Russia seize Ukraine warships in 2018 Protestors set up paper ships on the tires during their rally near of Russian embassy building in Kiev EPA Protests break out after Russia seize Ukraine warships in 2018 Smoke from a flare thrown by a protester is seen during a rally against the seizure by Russian special forces of three of the Ukrainian navy ships, which Russia blocked from passing through the Kerch Strait into the Sea of Azov in the Black Sea, in front of the Russian embassy in Kiev, Ukraine November 25, 2018. REUTERS/Gleb Garanich GLEB GARANICH Reuters Protests break out after Russia seize Ukraine warships in 2018 A man extinguishes a burning car of the embassy of Russia Reuters Protests break out after Russia seize Ukraine warships in 2018 Paper boats are seen placed during a protest Reuters Protests break out after Russia seize Ukraine warships in 2018 epa07190225 Ukrainian policemen guard the Russian embassy building with smoke behind during Ukrainian activists rally in Kiev, Ukraine, 25 November 2018. Russia has seized three Ukrainian vessels amid their leaving the Kerch Strait; Ukrainian President Petro Poroshenko is gathering the Military Cabinet over the incident. The two small-sized 'Berdiansk' and 'Nikopol' armored artillery boats have come under enemy fire and are now dead in the water. The 'Yany Kapu' tugboat has forcibly been stopped. The vessels have been captured by special forces of the Russian Federation, the press service of Ukraine's Navy said on Facebook on Sunday evening. The Ukrainian Navy also reported the number of the Ukrainian servicemen wounded in the incident grew to two persons as Ukrainian media report. EPA/STEPAN FRANKO EPA-EFE/STEPAN FRANKO EPA Protests break out after Russia seize Ukraine warships in 2018 epa07190217 A protestor throws a smoke grenade at the building during their rally near the Russian embassy in Kiev, Ukraine, 25 November 2018. Russia has seized three Ukrainian vessels amid their leaving the Kerch Strait; Ukrainian President Petro Poroshenko is gathering the Military Cabinet over the incident. The two small-sized 'Berdiansk' and 'Nikopol' armored artillery boats have come under enemy fire and are now dead in the water. The 'Yany Kapu' tugboat has forcibly been stopped. The vessels have been captured by special forces of the Russian Federation, the press service of Ukraine's Navy said on Facebook on Sunday evening. The Ukrainian Navy also reported the number of the Ukrainian servicemen wounded in the incident grew to two persons as Ukrainian media report. EPA/STEPAN FRANKO EPA-EFE/STEPAN FRANKO EPA

But without a resolution to the blockade, workers say they will be forced to wind down activities within the next few days. Parts of the neighbouring Berdyansk port were already reported idle on Thursday.

“If the blockade continues another week, real damage will be done,” said Marina Pereshevalevo, a trade union leader in the port. “I’ve worked here 32 years and it breaks my heart.”

Pressure has been building in the region since the outbreak of the war in 2014.

The port lost several key customers, especially coal mines, and a mainline railway when local territory fell to Russian-backed separatist forces. As a result, the port is operating below half capacity. If before the port handled a wide range of exports – coal, metal, grain, clay – now metal products from Mariupol’s three metal plants account for the vast majority (75 per cent) of its goods.

‘I’ve worked here 32 years and what’s happening breaks my heart,’ says Marina Pereshevalevo

But the real troubles began with the building of a bridge from annexed Crimea to the Russian mainland in 2016.

Ukraine has never been happy with the bridge, but has also complained about construction getting in the way of shipping. In March 2018, with the bridge nearing completion, it struck an early blow by detaining a Crimean fishing ship sailing under a Russian flag. (Kiev refuses to recognise the annexation.) And Russia responded predictably enough by arresting several Ukrainian ships.

Then, on 15 May, Russian President Vladimir Putin opened his bridge to a great fanfare, driving a big orange truck across it. Within two days, Russian border guards switched to a harsher regime of checking Ukrainian ships. Moscow said they were security checks, but in some cases, the checks lasted 10 days.

The new bridge also introduced a new height restriction of 35m, instead of the 38m originally projected, meaning many of the larger boats using the Ukrainian ports could no longer pass.

Russian ship rams into a Ukrainian navy tugboat

Ukraine asserts that its ships have been demonstratively targeted by Russia in order to exert military control of the Azov Sea.

“They’ve only ever stopped Ukrainian boats, and they’ve always done it in a most provocative way, ” says Maria Podybailo, a history lecturer turned military support activist in town. “Some of the crews were forced to strip naked, supposedly because they may have had nationalist tattoos.”

Russia insists any delays to the passage of vessels were down to the weather. It denies restricting shipping.

Since the skirmishes began, Mariupol has seen its traffic decrease by about 30 per cent. This equates to millions of dollars in trading losses.

The situation remains delicate, but Ukraine’s response in the last week has been forceful, declaring martial law in 10 border regions, including the Azov Sea.

It is unlikely to help the situation, says the local industrialist and politician Serhiy Taruta, nor reassure potential clients that Mariupol is open for business.

As the man who governed Donetsk region when it descended into war in 2014, Taruta says he knows something about crisis management. He tells The Independent the Kiev government is repeating many of the same mistakes it made back then.

“We aren’t in any position to fight Russia, but for some reason we insist on testing them. We didn’t need to arrest the Crimean boat. And we could have sent the three military ships by train last weekend. Instead we tried to make a point by sailing through Kerch. It seems nobody in the Ministry of Defence did a proper analysis of what might happen.”

Mr Taruta, a critic of the Ukrainian president Petro Poroshenko, said he believed Moscow had concluded Poroshenko was “incapable of compromise”, and would play politics with military escalation. For this reason, it was unlikely Moscow would start the major offensive that Poroshenko has been warning; the threat of a new military operation focused on Mariupol was “no more than two or three per cent”, he said. But if it happened, the Ukrainian front line would not stand a chance, martial law or no martial law. Soldiers are already demoralised by “years of static trench warfare”.

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Rather than upping the militaristic ante, Ukraine’s government should focus on lobbying for an “international forum” to regulate merchant shipping in the region, he says.

Taruta’s positions are unpopular with Mariupol’s army of military activists.

According to Maria Podybailo, Taruta’s “relaxed attitude to the Russian threat” was one of the reasons Ukraine lost control of Donetsk. She said that she would not let the “same thing” happen again. The government in Kiev had taken a while to understand the threat this time, she added, and only after her “intense lobbying”. She says she believes the message “got through”. Kiev sent reinforcements and began working on a new naval base in Berdyansk.

Ukrainian authorities remain coy about the scope and size of that new base, but The Independent understands several small gunships are now located there. Some of those ships came by the sea route, through Kerch, but others were, indeed, transferred by rail.

What the base is not is a major military threat, given the dozens of large Russian battleships located in and around the Azov Sea.

And that, says trade union leader Pereshevalevo, is what makes Mariupol’s 3,000 port workers very worried about their futures.

“We’ve come through a war together, but this is is the hardest thing yet,” she said.