"This terrible war crime cannot go unanswered," Robert Habeck, vice chancellor and economy minister, told German newspaper Bild the day after the bodies of nearly 300 civilians were found in mass graves after Russian troops withdrew, local Ukrainian officials said.

"I think that a strengthening of sanctions is called for. That's what we are preparing with our EU partners," Habeck added.

German Foreign Minister Annalena Baerbock said the images from Bucha were "unbearable".

"Putin's frantic violence is wiping out innocent families and knows no bounds," she wrote on Twitter, adding that those responsible for war crimes must be held to account.

"We will strengthen the sanctions against Russia and further support the defence of Ukraine," the minister said.

Ukraine's Foreign Minister Dmytro Kuleba called Bucha a "deliberate massacre" and urged G7 countries to impose "devastating" sanctions immediately.

Meanwhile, NATO Secretary General Jens Stoltenberg said Saturday that the killings of civilians in Bucha, Ukraine are "horrific."

"It is a brutality against civilians we haven't seen in Europe for decades, and it's horrific and it's absolutely unacceptable," Stoltenberg told CNN, referring to killings in the town retaken from Russian forces.