Ukrainian forces build second line of defence around major port and industrial centre as reports circulate of rebel buildup of fighters and weaponry in area

A plume of smoke rose above the horizon as a group of Ukrainian soldiers dug trenches near the village of Berdyansk outside Mariupol, seemingly oblivious to the mortar fire going off like popcorn over the crest of the hill.

The sun playing off the gentle waves of the Azov Sea seemed to belie the threat that the coastal city now faces as a possible next target for Russian-backed rebels who captured the strategic town of Debaltseve last week despite a ceasefire that came into effect on 15 February.

Vasily Ostapchuk laughed that he and his comrades from the 37th brigade had got used to the frequent shelling near the second line of defence that Kiev’s forces are building as part of a campaign to protect the city should it come under attack.

“They’re pulling together their forces and getting ready for something. First it was Debaltseve, now it’s Mariupol,” he said. “But it would be a crime to give up this city.”

After the capture of Debaltseve, which sits at both a road and rail junction connecting the capitals of the self-declared Donetsk and Luhansk people’s republics, fears have risen that Mariupol, a vital transport and economic hub, could be next to fall.

Following his announcement on Tuesday that British military personnel will be deployed to Ukraine as advisers, David Cameron said Mariupol was a likely target for pro-Russia forces in the wake of Debaltseve. The British prime minister said that if the city were captured, it would trigger more sanctions against Moscow that would be “materially different” from previous rounds.

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“People will be looking at Mariupol as the next potential flashpoint, and if that were to happen I think the argument for further action would be overwhelming,” Cameron said. “I think that would be the view of countries like Poland, the Baltic states and many others.”

Other western diplomats have suggested an attack on Mariupol would put an end to the Minsk peace plan, which has been troubled by frequent ceasefire violations, including the capture of Debaltseve, and a delay in a planned withdrawal of heavy weapons to create a buffer zone. Although no Ukrainian soldiers were killed on Wednesday for the first time in weeks, Russian armour has continued to enter Ukraine, much of it near Mariupol, military spokesman Andriy Lysenko said.

Pro-Russia forces deny any intention to take Mariupol, a position reiterated on Tuesday by Eduard Basurin, the deputy head of the Donetsk militia.

“We are not mounting an attack near Mariupol and we don’t plan to do so. We are carefully fulfilling the document signed by the head of the republic,” Basurin said, referring to the Minsk peace deal. “We stopped at the positions we held on 15 February.”

Also on Tuesday, however, the Donetsk militia told the Russian state news agency RIA Novosti of new gains bringing them closer to Mariupol, bragging that its forces had “liberated the [nearby] villages of Pavlopil and Pyshchevyk” on Monday night, and that “the national guard wasn’t able to dig in there”.

Photographs published on pro-Russia websites showed fighters standing outside the entry signs to both villages holding Kalashnikov assault rifles and a rocket-propelled grenade launcher.

A Ukrainian military spokesman said both villages were in the buffer zone established by the Minsk agreement and had not been held by Kiev’s forces.

Mariupol would be vital to the economy of any state or autonomous entity that rebels might establish in eastern Ukraine. With a population of 460,000 it is the second-largest city in the Donetsk region and the centre of eastern Ukraine’s metallurgical industry.

Facebook Twitter Pinterest The Azovstal steel works in Mariupol Photograph: Evgeniy Maloletka/AP

Much of the coal mined in the Donetsk and Luhansk regions has traditionally gone to fuel the country’s two largest steel-making plants in Mariupol, or has been exported via Mariupol port, the largest in eastern Ukraine.

Many in the city believe that the Russian president, Vladimir Putin, wants to create a land corridor to annexed Crimea, which would require conquering not only Mariupol, but also wide swaths of the Zaporizhia and Kherson regions, which have not yet seen large movements of pro-Russia forces.

Mariupol residents say the local population is evenly split between those who support Kiev and those who would like to join the breakaway Donetsk people’s republic. The pro-Russia side briefly held sway over Mariupol shortly after the start of the conflict, before Kiev’s forces drove them out in an operation in May.

Workers have been hanging hundreds Ukrainian flags across the city this week in an attempt to “show that Mariupol is a Ukrainian city,” said Galina Odnorog, a local activist.

APro-Russia forces have held the coastal town of Novoazovsk, about 20 miles east of Mariupol and less than 10 from the Russian border, since August. Rebels were holding Novoazovsk when the Guardian visited the town shortly after it was captured in a lightning offensive, but a few locals said at the time that they had seen Russian armour in the area.

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Mariupol has been steeling itself against attack since, building fortifications, setting explosives on bridges and mining the harbour. Few believe, however, that it could withstand an offensive backed by the Russian military.

A rocket attack that killed 30 people in eastern Mariupol on 24 January also raised concerns that the rebels could use their artillery to wage a war of attrition on the city, much as it did in Debaltseve. Since the rocket attack, city hall has carried out air raid drills with 560 residents of the eastern part of the city and is now teaching civil defence in other neighbourhoods.

On Wednesday, safety officials were instructing four dozen teachers from a local school on how to perform first aid and take cover during shelling. The doctor on hand, Sergei Yegorkin, said he had been at the busy market hit in the January attack and narrowly escaped death.

“I feel the danger every day,” Yegorkin said about the risk of more violence in Mariupol. “You can hear shelling every day from the eastern district. The windows shake.”

“We feel threatened and it’s frightening, but we have to rely on ourselves,” said teacher Alyona Taravik, adding that the school’s 623 students were drilled weekly on what to do in case of shelling.

The threat of an attack on Mariupol is not imminent, because rebel forces lack the armour and men needed to mount an operation, according to military spokesman Dmytro Chaly.

The area is, however, reportedly seeing a military buildup. According to Lysenko, pro-Russia forces have been redeploying troops and equipment. Kiev said on Friday that 20 Russian tanks, 10 missile systems and busloads of fighters had crossed the border and headed to Novoazovsk.

Andrei Dyachenko of the Azov battalion, a volunteer fighting unit known for the neo-Nazi views of some of its members, said its reconnaissance had observed tanks, fighting vehicles and self-propelled artillery gradually reinforcing pro-Russia forces near Mariupol, with eight to 10 pieces of armour arriving each day for the past four days. Azov, which Dyachenko said has more than 1,000 fighters, has played a key role in the defence of the city since the original struggle with rebels in May.

Facebook Twitter Pinterest Ukrainian Azov batallion volunteers train at their base in Mariupol Photograph: Ivan Boberskyy/EPA

“We think they’ll attack at the end of March,” Dyachenko said. “If only the Donetsk people’s republic and Novorossiya rebels storm it, then it will be easy. If there are Russian forces, like the troops that are now building up in Rostov [near the border], then it will be harder to defend the city.

“British advisers are not a panacea, although it’s good that they’ll help,” he added.

Clashes between pro-Russia and Ukrainian forces have been increasing in the area since the ceasefire. The epicentre of the fighting has been the town of Shyrokyne, which by some estimates has been three-quarters destroyed by frequent shelling.

In the days before ceasefire negotiations began in Minsk, Azov took the town in what president Petro Poroshenko said was a counter-offensive to re-establish the previous demarcation line. According to Chaly, the main reason for taking Shyrokyne, which he said had previously not been controlled either side, was to push the rebels’ Grad multiple rocket launcher systems out of range.

The hospital in Novoazovsk has registered a recent rise in battle casualties. Since 10 February, it treated 39 battle injuries, including 16 since Sunday, Agence France-Presse reported. Four of those in the hospital died from their wounds in recent days, and a rebel commander said three of his men had been killed on Monday.

In Berdyansk on Tuesday, the Guardian heard intensive incoming and outgoing mortar fire in Shyrokyne, and heavy machine gun fire. Soldiers were also moving up tanks to the defensive positions being built in the village. Kiev has said it will not withdraw heavy weaponry until the rebels stop firing, and both sides have accused the other of breaking the ceasefire.

The commander of the Donbass volunteer battalion troops in the Mariupol area, which are currently holding Shyrokyne, said his men had exchanged fire with rebels, destroying a fuel truck and a flak gun.

“If they come to visit with mortars, then we will answer them with mortars,” said the commander, who would give only his call sign Swat.