Evidence of possible civilian killings around Kyiv has emerged as the Russian army has pulled back from the capital in the face of ferocious resistance from Ukrainian forces.

AFP reporters saw at least 20 bodies, all in civilian clothing, strewn across a single street in the town of Bucha on Friday. One had his hands tied behind his back with a white cloth, and his Ukrainian passport left open beside his body.

And a Ukrainian official said 57 bodies had been buried in a mass grave in the town outside the capital, showing AFP a slit trench were the bodies lay.

Roughly ten bodies were visible, either unburied or partially covered by the earth.

"You can't help but see these images as a punch to the gut," Secretary of State Antony Blinken told CNN a day after horrific footage from Bucha, recently retaken from Russian forces, was widely aired.

"This is the reality of what's going on every single day as long as Russia's brutality against Ukraine continues," Blinken said.

NATO Secretary General Jens Stoltenberg said the killings of civilians in Bucha 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.

Stoltenberg also said he was not "too optimistic" about Russia's claim to be pulling troops away from Kyiv.

"What we see is not a withdrawal, but we see that Russia is repositioning its troops," he told CNN.

"We should not in a way be too optimistic because the attacks will continue and we are also concerned about potential increased attacks," Stoltenberg said.