KYIV — On the night of 3 April, a swarm of Russian drones attacked Kharkiv, in Ukraine's northeast.

The country's second-largest city has been targeted almost incessantly since the start of Russia's full-scale invasion.

But this time was worse than usual, because, when rescue workers arrived at the scene, there was a second strike. Three of them were killed.

The following Friday, it happened again when Russian missiles hit Zaporizhzhia, a major city in Ukraine's southeast.

Rescuers and journalists rushed to the scene, and then two more missiles hit.

In total, four people were killed and more than 20 were wounded, including two local journalists.

Both the Kharkiv and the Zaporizhzhia attacks employed a technique called "double-tap" — when an initial air strike is followed by a second attack, killing rescuers trying to help the injured.

Russia has carried out this kind of repeat attack before. Ukrainian President Volodymyr Zelensky said a strike on a building in the southern city of Odesa in March that killed 20 was a double-tap, and called it a "despicable act of cowardice".

But Ukrainian officials say they have seen an increase in the use of such attacks.

Oleh Synehubov, the governor of Kharkiv region, told Ukrainian media that Russia has started incorporating repeat strikes on its targets "day and night".

"The occupiers are using the tactic of double-taps to hit civilian rescuers and other workers who are there first to arrive at the scene," he said.

On Thursday, the UN Office for the Coordination of Humanitarian Affairs said it had seen a "particularly troubling pattern" of double taps in recent weeks. It called them "cruel" and "unconscionable" and urged for them to stop.

Rescuers from Ukraine's State Emergencies Service DSNS often arrive to deal with the aftermath of Russian attacks before everyone else, and they also say the threat of repeat strikes is becoming ever more frequent.

"Unfortunately, the tactic of double-tapping has been used increasingly often recently. This is difficult to comprehend. The Russians have no right to do this," DSNS spokesman Oleksandr Khorunzhy told the BBC.

"They know perfectly well what they're doing, and not just to rescuers, police officers, utility workers or medics. This affects ordinary civilians," Khorunzhy said.

He called double-taps "utterly immoral".

"I can't wrap my head around this, it's just inhuman," he said. "They see perfectly well that unarmed rescuers are the first to arrive at the sites of their attacks."

The DSNS says 91 of its rescuers have been killed and more than 340 injured since the Russian full-scale invasion in February 2022.

Kira Oves was one of the journalists injured in the Zaporizhzhia double-tap attack.

She told the BBC that she had just arrived at the scene of a strike when she heard a whistling sound.

"A colleague shouted: Incoming! On the ground!"

She crouched and heard a massive explosion. She only realised that she was wounded after she felt blood streaming down her face. She and her colleague ran to what they thought was a safe place — only to hear another "enormous blast".

"I knelt down, and a policewoman rushed over to me to help close the wound. Another police officer bandaged my head to stop the bleeding. From the shouting we found out that another journalist had been wounded much more badly," Kira said.

She had not expected two more strikes to follow the one she had gone to report on.

Ukrainians targeted by double-tap attacks accuse Russia of trying to crush their fighting spirit and resilience.

They also worry that no medic or emergency worker would be prepared to risk coming to their rescue for fear of coming under fire, too.

But there are other pragmatic reasons for Russia's strategy too.

"If you abandon any pretence at subscribing to international humanitarian law and even commonly accepted humanitarian norms, then there is a sound military rationale for double-tap attacks because you target high-profile targets," says Keir Giles, director of the Conflict Studies Research Centre, a British think-tank.

There is a limited supply of first responders such as medics, emergency personnel and fire crews and they are difficult to replace.

"If you take them out by the second attack on the same spot, at exactly the time when they've congregated to help the victims of the first attack, you're actually achieving quite a lot."

Karolina Hird, an analyst at the US-based Institute for the Study of War, says double-tap attacks targeting rescue workers could, and likely do, constitute a war crime.

"If, indeed, Russian forces are found to be targeting specifically and intentionally these sections of the population — that would constitute a violation of international humanitarian law and the general rules and norms of armed conflict," Ms Hird told the BBC.

A spokesman for the International Committee of the Red Cross told the BBC that all parties involved in a conflict had an obligation to "respect and protect" both the wounded and those who cared for them.

If one party chooses to re-attack the same target, international humanitarian law requires them to take "particular care" when first responders are trying to evacuate the wounded, the ICRC said.

Russia adopted the same strategy previously in Syria, where its troops fought against anti-government rebels between 2015 and 2017. In that case it targeted rescuers from the White Helmets, a civil defence group.

"We have documented hundreds of such cases since Russia intervened in 2015," the White Helmets chief Raed al-Saleh says. "Frankly, the Russian army has no morals when it comes to military operations. It focuses on breaking the will of civilians."

Russia has not explicitly disavowed double-taps, but it has repeatedly denied it aims for civilian targets.

"Our military does not hit social facilities and residential neighbourhoods and does not hit civilians," Kremlin spokesman Dmitry Peskov said in January.

But those in towns and cities across Ukraine see the opposite is the case.

"They're using double-taps to kill as many of us as possible," says Kira Oves, the journalist wounded in one such attack in Zaporizhzhia. "But they'll fail, because Ukrainians are survivors. We shall win." — BBC