Kyiv, Ukraine (AFP) — At least six postal workers were killed while 17 were wounded after Russian missile strikes hit a mail depot in Ukraine's northeastern Kharkiv region, officials said.

The strike in the northeastern Kharkiv region on Saturday came as Kyiv declared its positions in the embattled city of Avdiivka were "protected" despite Russian attacks, while Moscow said it had downed Ukrainian missiles targeting the Crimean Peninsula.

The six killed in the depot attack were all workers at the Ukrainian postal operator Nova Poshta in Korotych, a village on the outskirts of Kharkiv city, regional governor Oleg Sinegubov said.

"The victims, aged between 19 and 42, received shrapnel wounds and blast injuries," he said.

Of the injured being treated in hospital, seven were in a serious condition, according to Sinegubov, who said "doctors are fighting for their lives."

The regional prosecutor's office later updated the number of injured to 17.

President Volodymyr Zelensky shared a video on social media of what appeared to be a heavily damaged warehouse surrounded by rubble and a container with the Nova Poshta logo.

Sergiy Nozhka, who works for Nova Poshta, described the condition of some his colleagues as "mild to moderate severity," adding that "there are some people in a very serious condition."

He said that a rocket "flew into the neighboring depot, but at ours too ­— the windows and shutters flew out. This is not the first time."

According to the prosecutor's office, Russian forces in the Belgorod region north of Kharkiv fired S-300 missiles, two of which hit the warehouse.

"Debris analysis continues at the site in order to establish the exact number of injured and dead," office spokesperson Dmytro Chubenko told Ukraine's state broadcaster Suspilne.

Separate Russian attacks on villages near the war-battered Ukrainian city of Bakhmut killed at least two people on Sunday, officials said.

Both Kyiv and Moscow are preparing for a grueling winter ahead, as Ukraine warns of renewed strikes on its energy infrastructure and Russia contends with a Ukrainian counteroffensive to regain territory.

In the eastern Donetsk region, Ukraine's positions around the frontline city of Avdiivka were "protected", Zelensky said in his evening address Sunday.

The city has been the center of intense fighting in recent weeks as each side struggles to make advances. Ukraine's general staff said on Friday that Russia had stepped up its military assault on Avdiivka in an ongoing bid to encircle and capture it.

"The Avdiivka and Maryinka directions are particularly tough", Zelensky said. "Numerous attacks by Russians. But our positions are protected."

Avdiivka has been a symbol of Ukrainian resistance since 2014, after it briefly fell to Russian-backed separatists.

It lies just 15 kilometers (nine miles) from the Moscow-held city of Donetsk, capital of the Donetsk region that Russia said last September it was annexing.

Ukrainian soldiers had been bracing for a new assault after a failed Russian offensive earlier this month using columns of armored vehicles and tanks from three sides.

Built around a huge coke plant, Avdiivka had a pre-war population of around 30,000 people.

Around 1,600 remain, according to local authorities, living in basements converted into bomb shelters.

The city center has been all but destroyed through daily Russian artillery shelling and a months-long aerial bombing campaign.

Also on Sunday Russian forces shot down three missiles targeting the Crimean Peninsula, which Moscow annexed from Ukraine in 2014, a Russian official said.

The peninsula is crucial to Russia's offensive, both for supplying troops in southern Ukraine and for carrying out missile strikes from the sea.