Ukraine said Wednesday it reclaimed more territory from Russia in the south and welcomed the delivery of Western air defence systems that Kyiv said would usher in a "new era" after mass strikes from Moscow.

Russia for two days pummelled Ukraine with missiles, damaging energy facilities nationwide, in attacks that President Vladimir Putin said were retaliation for a deadly explosion at the Crimea bridge.

Russia's FSB security service said Wednesday it detained eight suspects over the blast that ripped through the road and rail bridge connecting Crimea to the rest of the country.

But it also claimed to have foiled two more attacks that Ukrainian special services allegedly planned to carry out on Russian territory.

NATO chief Jens Stoltenberg said Wednesday after Russia's missile barrage that Ukraine's Western backers were looking to provide Kyiv with more air defences to protect against Russia's "indiscriminate" attacks across the country.

"We will address how to ramp up support for Ukraine and the top priority will be more air defence for Ukraine," Stoltenberg said at the start of a meeting by Ukraine's allies on arms supplies to Kyiv.

Putin has vowed a "severe" response to any further attack on Russia and what Moscow considers to be its territory, including the Crimea peninsula that it annexed from Ukraine in 2014.

Despite warnings from the Kremlin, Kyiv has vowed to retake the peninsula as well as four regions in Ukraine's east and south that Moscow says are now part of Russia.

Kyiv said Wednesday that it had retaken five more settlements in the southern region of Kherson -- one of the four territories Moscow said it annexed in late September -- in the latest setback for Russia's campaign.

Putin 'miscalculated'

The presidency added, however, that Russian forces were striking back and had continued shelling Ukraine's positions "along the entire contact line".

The Russian military meanwhile said it had fended off Ukrainian attacks in the eastern Donetsk, Lugansk and Kharkiv regions.

And Russian strikes on the frontline town of Avdiivka killed at least eight people at a market, the Ukraine-appointed chief of the region said.

"There is no military logic to this kind of shelling, only the unbridled desire to kill as many of our people as possible and intimidate others," the Donetsk governor Pavlo Kyrylenko said on social media.

The Ukrainian army announced its counter-offensive in the south in late August.

After regaining almost full control of the northeastern region of Kharkiv, Ukrainian forces recently claimed more gains on the eastern and southern fronts.

Faced with mounting setbacks since September, the Russian president announced the mobilisation of hundreds of thousands of reservists to join the fighting in Ukraine.

With the Crimea bridge blast, Russia also lost a vital transport link for moving military equipment for Russian soldiers fighting in Ukraine.

US President Joe Biden said Tuesday that he believes his Russian counterpart had "miscalculated" the situation in Ukraine and underestimated the ferocity of Ukrainian defiance.

"He thought he was going to be welcomed with open arms, that this was the home of Mother Russia in Kyiv," Biden told CNN in a rare televised interview.

"I think he just totally miscalculated."

Mass graves discovered

After two days of nationwide Russian strikes that especially targeted Ukraine's energy infrastructure, leaving villages and towns without power and hot water, Ukraine said it had started receiving anti-aircraft defence systems from its Western allies.

"A new era of air defence has begun in Ukraine," Ukraine's Defence Minister Oleksiy Reznikov said on Twitter, announcing the arrival of Germany's Iris-Ts and the upcoming delivery of NASAMS from Washington.

He said he had met with Defense Secretary Lloyd Austin and General Mark Milley and discussed the "strengthening of the combat potential of the Ukrainian army", according to a tweet.

On Tuesday, Ukraine's President Volodymyr Zelensky called on the G7 club of wealthy nations to help Kyiv create an "air shield", warning that Russia "still has room for further escalation".

Ukrainian officials announced Tuesday the recovery of the remains of dozens of civilians found at mass burial sites in two towns in the eastern Donetsk region recently recaptured from Moscow's forces.

In Lyman, a railway hub retaken by Ukraine in early October, a forensic team dressed in protective gear was exhuming dozens of bodies, an AFP journalist saw.

"We have already found more than 50 bodies of both soldiers and civilians. We have one long trench -- a mass grave -- where we discovered bodies and body parts," regional governor Kyrylenko said.

Russian forces have been accused of numerous abuses -- torture, rape, extrajudicial executions -- in Ukraine, claims Moscow has repeatedly denied.