Ukraine is investigating more than 25,000 suspected war crime cases since Russia's invasion five months ago.
Yuriy Bilousov, head of the war crimes department of the prosecutor general's office, said 135 people had already been charged, although just 15 of those are now in custody and 120 are still at large.
He told Reuters news agency 13 cases have been submitted to courts and seven verdicts have been issued since the 24 February invasion was launched by Vladimir Putin.
In May, a 21-year-old captured Russian soldier became the first person to be convicted in a war crimes trial in Ukraine. He was sentenced to life in prison for killing an unarmed civilian.
"Sometimes we've been asked why we prosecute such ... low-ranked officers. It's just because they are physically here," Mr Bilousov said.
“If generals were here physically and we were able to capture [them], we would definitely prosecute generals.”
Reports suggested many civilians had been left dead in the street in Bucha, some with their hands bound, and satellite imagery revealed what appeared to be the presence of a mass grave in the town.
US president Joe Biden has branded Mr Putin a “war criminal” and Ukrainian president Volodymyr Zelensky referred to the situation in Bucha as an act of genocide.
The first independent investigation into allegations of Russian violations of humanitarian law in Ukraine, published in April, found evidence of multiple war crimes.
The preliminary report by the Organisation for Security and Cooperation (OSCE) in Europe found that the bombardment of a children’s hospital and theatre in the besieged port city of Mariupol amounted to war crimes, as did the forcible removal of thousands of Ukrainians to Russia.
Investigators said there were “credible reports” of civilians being detained by Russian troops and subjected to ill treatment which amounted to torture.
