ZAPORIZHZHIA, Ukraine — They sneak down darkened alleys to set explosives. They identify Russian targets for Ukrainian artillery and long-range rockets provided by the United States. They blow up rail lines and assassinate Ukrainian officials they consider collaborators with the Russians.
Slipping back and forth across the front lines, the guerrilla fighters are known in Ukraine as partisans, and in recent weeks they have taken an ever more prominent role in the war, rattling Russian forces by helping deliver humiliating blows in areas they occupy and thought to be safe.
Increasingly, Ukraine is taking the fight against Russian forces into Russian-controlled areas, whether by using elite military units, like the one credited on Tuesday with a huge explosion at a Russian ammunition depot in the occupied Crimean Peninsula, or by deploying an underground network of the partisan guerrillas.
Last week, Ukrainian officials said, the partisans had a hand in a successful strike on a Russian air base, also in Crimea, which Moscow annexed eight years ago. It destroyed eight fighter jets.
“The goal is to show the occupiers that they are not at home, that they should not settle in, that they should not sleep comfortably,” said one guerrilla fighter, who spoke on condition that, for security reasons, he only be identified by his code name, Svarog, after a pagan Slavic god of fire.
In recent days the Ukrainian military made Svarog and several other of the operatives available for interviews in person or online, hoping to highlight the partisans’ widening threat to Russian forces and signal to Western donors that Ukraine is also successfully rallying local resources in the war, now nearly 6 months old. A senior Ukrainian military official familiar with the program also described the workings of the resistance in detail.
Their accounts of attacks could not be verified completely but aligned with reports in the Ukrainian media and with the descriptions of Ukrainians who had recently fled Russian-occupied areas.
Svarog and I met over lemonade and cheese pastries at a Georgian restaurant in Zaporizhzhia, a city under Ukrainian control about 65 miles north of the occupied town of Melitopol, where he operates.
He spoke with intimate knowledge of partisan activities, providing a rare glimpse into one of the most hidden aspects of the war.
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The Ukrainian military began training partisans in the months before the invasion, as Russia massed troops near the borders. The effort has paid off in recent weeks as Ukrainian forces are pressing a counteroffensive in the south.
Insurgent activity is now intensifying, as the resistance fighters strike stealthily in environs that they know intimately, using car bombs, booby traps and targeted killings with pistols — and then blending into the local population.
Before the war, Svarog occasionally joined weekend training with Right Sector and National Corps, a branch of the Azov movement, both of which are aligned with paramilitary units in Ukraine. They were just two of dozens of organizations running military training for civilians throughout Ukraine during the eight-year war with Russian-backed separatists.
Svarog said he was among the trainees in these public programs. Behind the scenes, Ukraine’s Special Operations Forces were forming a more structured, and secret, program that included instruction on sabotage and explosives and stashing caches of weapons in anticipation of Russia’s attack.
After the invasion, Svarog said, he was directed to a cache in a storage shed outside Melitopol, where he found slabs of high explosives, detonators, Kalashnikov rifles, a grenade launcher and two pistols equipped with silencers.
Melitopol, the southern Ukrainian town where Svarog operates, has since emerged as a center of the resistance. He recounted the careful casing of targets, followed by attacks.
By Saturday, partisans had struck with explosives seven days in a row, according to the town’s exiled mayor, Ivan Fedorov, who boasted of the achievement to Ukrainian media as part of the more public embrace of partisan operations by officials.
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The attacks have been going on for several months. In one attack this spring, Svarog said, he and several members of the cell in Melitopol sneaked through the town at night to booby-trap a car in the parking lot of a Russian-controlled police station.
Carrying wire cutters, tape and fishing line, the fighters moved through courtyards and back alleys to avoid Russian checkpoints.
They first cut an electrical wire, blacking out a streetlight, then dashed quickly into the darkness where they planted a bomb, wrapped in tape with the sticky side facing outward, into a wheel well. The fishing line was taped both to the inside of the wheel and to a detonator, rigging the bomb to explode when the wheel turned.
“Anybody who would drive that car would be a traitor,” Svarog said. “Nobody there is keeping public order.” The bomb killed one policeman and wounded another.
In a strike last week, he said, his cell booby-trapped the car of Oleg Shostak, a Ukrainian who had joined the Russian political party United Russia in Melitopol. The insurgents targeted him because they suspected him of tailoring propaganda to appeal to local residents.
Svarog, who said he did not take part in this particular mission, said his team placed a bomb under the driver’s seat of the car, rigged to explode when the engine started.
Mr. Shostak was wounded in the explosion but survived, said Mr. Fedorov, the exiled mayor of Melitopol. The attack was separately reported by Ukrainian authorities and described by displaced people leaving Melitopol through a checkpoint to Ukrainian territory on Sunday.
Whether targeted people survive or die in the attacks, partisans say, is less important than the signal they send with each strike: You are never safe.
Separately, two partisans operating in occupied southeastern Ukraine and interviewed by video call described a branch of the underground called Yellow Ribbon, which carries out nonviolent actions such as posting leaflets and spray-painting graffiti.
The bases on Ukrainian territory where operatives are trained are moved constantly to avoid discovery, according to a senior Ukrainian military official. The official spoke on condition of anonymity to discuss sensitive military information.
Each operative each has a different a role to play, the official said: scouting a target, gathering intelligence on the movements of a target, and carrying out an attack. Individual cells are kept separate and do not know one another, lest a detained partisan reveal identities under interrogation.
Two entities within the military are responsible for overseeing operations behind enemy lines, the official said: the military intelligence service, known as HUR, and Ukraine’s Special Operations Forces. An interagency task force oversees the operations of both the intelligence agency and Special Operations Forces branches of the underground, what is known as the Resistance Movement, or Rukh Oporu in Ukrainian.
The official described a poisoning in the Zaporizhzhia region that killed around 15 Russian soldiers and the sabotage of a grain elevator in the Kherson Region that prevented Russian forces from stealing 60,000 tons of grain. Neither operation could be independently verified.
Partisans were also behind an explosion on Saturday that disabled a railroad bridge connecting the city of Melitopol to Crimea, halting the supply of military equipment coming into the Zaporizhzhia region.
“They’re frightening people, these Ukrainian partisans,” the official said. “But they’re frightening only for the occupiers.”
And for those the partisans consider to be traitors, too.
The Ukrainian underground in occupied territory considers policemen, municipal and regional government employees and teachers who agree to work under the Russian educational curriculum as collaborators, according to Svarog and another partisan using the nickname Viking, who was interviewed in an online video call. They said they do not see doctors, firefighters and employees of utility companies as traitors.
Teachers are a focus now, with schools scheduled to open in September.
“The Russians want to teach by their program, not the truth,” Viking said. “A child is vulnerable to propaganda and if raised in this program, will become an idiot like the Russians,” he said. “A teacher who agrees to teach by the Russian program is a collaborator.”
Partisans will not attack teachers, he said, but have sought to publicly humiliate them in the leaflets they often post on utility poles with dark warnings for collaborators, as part of the guerrillas’ psychological operations.
One went up recently, he said, with the names and photographs of principals planning to open schools in September.
It said simply: “For collaborating with the Russians, there will be payback.”
Yurii Shyvala contributed reporting from Zaporizhzhia, Ukraine, and Michael Schwirtz from Odesa.