A serviceman takes photo of destroyed Russian armoured vehicles in Bucha, Kyiv, Ukraine, on Saturday. Reuters

As Ukraine claimed its forces had retaken all areas around Kyiv and Chernigiv city, as evidence emerged of possible civilian killings in areas the invading forces have been occupying.

The mayor of a liberated town said 300 residents had been killed during a month-long occupation by the Russian army, and victims were seen in a mass grave and still lying on the streets.

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Series of blasts heard in Ukraine's southwestern city Odessa

Retreating Russians leave many mines behind: Ukrainian President

AFP reporters saw at least 20 bodies, all in civilian clothing, strewn across a single street in the town of Bucha near the capital and the body of a missing photographer was discovered in a nearby village.

Ukraine's troops have retaken more than 30 towns and villages around Kyiv, Ukrainian officials said on Saturday, claiming complete control of the capital region for the first time since Russia launched its invasion.

Ukrainian service members hold their weapons near the village of Kozarovychi, Kyiv, Ukraine. Reuters

At Bucha, a town neighbouring Irpen, just 37km (23 miles) northwest of the capital, the media journalists saw bodies lying in the streets, and the hands and feet of multiple corpses poking out of a still open grave at a church ground.

After five weeks of fighting, Russia has pulled back forces that had threatened Kyiv from the north to regroup for battles in eastern Ukraine.

"The whole Kyiv region is liberated from the invader," Ukrainian Deputy Defence Minister Hanna Malyar wrote on Facebook. There was no Russian comment on the claim, which Reuters could not immediately verify.

A burned car is seen on the highway, amid Russia's invasion of Ukraine, in Myla, Kyiv, Ukraine. Reuters

President Volodymyr Zelenskiy warned in a video address: "They are mining all this territory. Houses are mined, equipment is mined, even the bodies of dead people." He did not cite evidence.

British Foreign Secretary Liz Truss said she was "appalled by atrocities in Bucha and other towns in Ukraine".

"Reports of Russian forces targeting innocent civilians are abhorrent. The UK is working with others to collect evidence and support @IntlCrimCourt war crimes investigation. Those responsible will be held to account," Truss tweeted late Saturday.

The International Criminal Court has already opened a probe into possible war crimes committed in Ukraine, and several Western leaders, including US President Joe Biden, have accused Russia's Vladimir Putin of being a "war criminal".

Ukrainian servicemen ride on a tank vehicle outside Kyiv, Ukraine, on Saturday. AP

Zelensky has also accused Russian soldiers of planting mines and other booby traps as they withdraw from northern Ukraine.

"They are leaving behind a complete disaster and many dangers.... Firstly, the air strikes may continue. Secondly, they are mining the whole territory. Mining houses, equipment, even the bodies of people who were killed," he said in a video address Saturday, warning returning residents of tripwires and other dangers.

"We are moving forward. Moving carefully and everyone who returns to this area must also be very careful," he said.

While Russian forces appeared to be withdrawing from the north, a series of explosions were heard Sunday morning in the historic Black Sea port of Odessa, with an AFP journalist reporting columns of black smoke and flames visible, apparently in an industrial part of the strategic city.

Civilians cross a destroyed bridge in the village of Kozarovychi, in Kyiv region, Ukraine, on Saturday. Reuters

Anton Herashchenko, adviser to the interior minister, said Odessa was attacked from the air.

"Fires were reported in some areas. Part of the missiles were shot down by air defence," he wrote on Telegram.

Meanwhile, in an attempt to raise economic pressure on Russia, the Baltic states of Latvia, Estonia and Lithuania announced Saturday that they had stopped all imports of Russian natural gas.