‘The Russians are trying to obliterate us. You think that we’d agree to half-measures now?’

The sun was shining in Zaporizhzhia on Saturday and teams of city workers were out planting flowers on roadside borders. Market stalls were doing a healthy trade in everything from food and drink to electronics, and there were even a few cafes and bars open.

But the frontlines are barely a half-hour drive from this industrial city, and much of the region of which it is the capital is under Russian occupation. Underneath the calm exterior, most people here - and across the southern and eastern parts of Ukraine - are anxious about what lies in wait in the coming days and weeks.

Having failed miserably in his attempts to march speedily on the Ukrainian capital, Russian president Vladimir Putin withdrew troops from around Kyiv a week ago, leaving ruin, grief and fury in their wake.

The expectation now is that he will order a massive assault in the Donbas region of eastern Ukraine to capture more ground for Russia and make territorial gains he can present as a victory.

The Ukrainian government has suggested residents of the nearby Donbas region should evacuate immediately, while the mayor of neighbouring Dnipro also told women, children and the elderly to leave last week. But as the horrific missile attack on a crowd of civilians in Kramatorsk on Friday showed, Russia has no qualms about fighting beyond the battlefield, and people can face slaughter as they try to escape.

"The battle for Donbas will not just be a battle for that territory, it will be a battle for global security," said Ivan Fedorov, the mayor of Melitopol, a city south of Zaporizhzhia that came under occupation in the first days of the war.

Fedorov, who was kidnapped by Russian soldiers and eventually swapped in a prisoner exchange, now works from an office in Zaporizhzhia. After a week in which news of horrific war crimes in Bucha and other small towns near Kyiv has shocked the world, Fedorov's demeanour in an interview with the Observer was characteristic of a new Ukrainian resolve to continue fighting the Russian assault.

"We should not talk about half measures. Today, all the red lines are crossed. Thousands of peaceful citizens have been killed," said Fedorov, frequently breaking off to field calls on two different phones about continued evacuations from occupied Melitopol.

He reacted angrily to a question about whether Ukraine should still attempt to negotiate a peace settlement: "Bucha has been obliterated, fucking obliterated. And you think we're going to agree to half measures?"

This sentiment is widely shared and suggests the battle for Donbas could be long and bloody, involving a more focused and determined Russian attack force, as well as a rejuvenated and vengeful Ukrainian army, fighting on terrain where the war has been continuing for the past eight years.

In Zaporizhzhia, government buildings are protected with sandbags while police and sniffer dogs check arriving passengers at the railway station, and air raid sirens sound a few times each day. At the train station, evacuees from Mariupol and other towns occupied by Russia wait for onward travel to western Ukraine and safety.

A large billboard on the main street addresses hypothetical Russian occupiers: "Russian soldier! Ukrainians are defending their families! And what will you die for? Put your weapons down. Stay alive."

For now, Russia probably does not have the resources to launch full-scale attacks against cities such as Zaporizhzhia and Dnipro, but it is likely that Putin has not given up on trying to bring at least the eastern half of Ukraine under Moscow's control.

"They will concentrate on Donetsk and Luhansk regions - that is where we will see the most serious battles for now," deputy prime minister Iryna Vereshchuk told the Observer. "Beyond that, it will depend on how things go on the battlefield."

Fedorov said cities such as Zaporizhzhia and Dnipro had the advantage of having time to prepare for a spirited defence. "Melitopol wasn't ready," he said.

The Russian army took control of Melitopol and a number of other cities in the south of Ukraine in the first days of the war without much of a fight. A few days after the Russians took over Melitopol, a group of armed men Fedorov assumed were from Russia's security services arrived at his office and said he could keep working as long as he accepted Russian control over the city.

"They said Melitopol is Russia, and that we could keep doing what we were doing, but we should recognise that they are now in charge of the security of the city," he recalled.

Melitopol is a largely Russian-speaking city and many people there, including Fedorov, have relatives in Russia. But he said the vast majority of the city is now solidly pro-Ukrainian, and this has only intensified since the Russian invasion.

Fedorov, and almost everyone in his team, refused to cooperate with the Russians, he said, leading them to become ever more irate, especially when demonstrators with Ukrainian flags began taking to the streets.

"These rallies were the final straw for the Russians and they decided to take me prisoner. In broad daylight, they came to the social assistance centre we'd set up on the first day, where we gave out food and clothes, tied my hands together and put a bag over my head, and marched me out," he recalled.

The Russians appointed Halyna Danylchenko as the new mayor of the city, and she released a video appeal calling on people to welcome Russian rule. "Our main task right now is to adapt to the new reality, so that we can begin to live in a new way," she said.

At the same time, soldiers took Fedorov to one of Melitopol's pre-trial detention centres, now run by the Russians, and gave him four sheets of paper to sign, offering the resignation of his team and agreeing to hand over power. He said he signed them, as he realised they had no legal power and he was assured he would be able to leave if he did so. After he signed, he was left on his own for a day and a half, then late in the evening of 7 March a new interrogator arrived, and an overnight interrogation began, with a group of armed men standing over him, Fedorov recalled.

Fedorov himself was not subjected to physical abuse, but he said it was clear that others in the police station were.

"They were playing psychological games. He was being more or less polite, but in the neighbouring cell I could hear screams - it was very audible that they were breaking someone's fingers," he said.

Fedorov said his conversations with his jailers were surreal. "I asked them to explain what they were doing in my city, in my country. They said: 'We're coming to emancipate the Russian language.' I said: 'In Melitopol, 95% of people speak Russian - there is not such a problem.'"

The guards also said they had come to free Ukraine from Nazis, and to stop second world war veterans being beaten up, Fedorov claimed.

"There are 34 living veterans in Melitopol; I know them all, we have good relations, we congratulate each other on 9 May, we meet on 23 October on the day of liberation of Melitopol - this is our history. Not only do we not beat them up, we are very thankful to them that we live under a peaceful sky, there is no question about that," said Fedorov.

After six days in captivity, Fedorov was swapped in a prisoner exchange, and since then has been based in Zaporizhzhia. After meeting president Volodymyr Zelenskiy, he then went on a tour of western Europe, bringing a message that the west should do more to support Ukraine.

"If there won't be peace in Ukraine, there will be war in Europe," is what Fedorov said he told French president Emmanuel Macron when they met in Paris.

In the week since Fedorov went to Paris, news of war crimes in Bucha and elsewhere has made it much harder to see any negotiated end to the conflict, at least for the foreseeable future.

As more and more stories of killings, rapes and other war crimes come to light, however, the optimism looks even more misplaced."We should approach this professionally, without emotion, and productively," said Rustem Umerov, a Ukrainian MP who is part of Kyiv's delegation to the negotiations.

But he also made it clear that Ukraine needed to continue inflicting losses on the battlefield in order to win ground in negotiations. "We understand that political power flows from the military situation," he said.

Fedorov said Ukraine would be foolish to stop until it had completely defeated Russia on the battlefield, otherwise it would only be buying time for a renewed offensive. "It is a mistake to try to predict a logical development of the situation - you need to take your paradigm and insert into it a sick man with a sick worldview and imperial ambitions," he said.

"You think he's going to be satisfied with some compromise? No, never. Only when he's fully defeated."