Over the summer, the number of civilians wounded by Russian attacks on the two cities increased from 70 in June to 212 in August, say Ukrainian police.
Sixteen people died in June, 20 in July and 15 in August.
Residential housing comes under attack as well as infrastructure, says Dyachenko, causing "very significant destruction".
The number of glide bombs launched by Russia jumped from 40 in June to 109 in July and 92 in August, according to data published by Ukrainian police and studied by the BBC.
One morning two weeks ago, on 25 August, nine glide bombs targeted Kramatorsk. Seven hit civilian apartment blocks, wounding 24 people including a two-month-old baby, Ukrainian police say.
Two days later, it was hit again, this time with seven glide bombs, and five bombs targeted Slovyansk.
Fitted with winglets so they can glide to their target rather than fall, glide bombs are considered one of the most effective and lethal weapons in Russia's arsenal.
Usually, they weigh 250 or 500 kg, but there have been cases of Russia deploying bombs that weigh 1,500 kg.
"The devastation they cause is simply incredible," says Dyachenko.
"Glide bombs are the big fear," another British volunteer, John Auckland Abbey, posted on social media.
"They are silent killers, and when you've been abruptly woken by one exploding, you know at least another one will follow. The question you ask yourself as you lie very still in the dark is: will the next one be closer?"
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