Ukrainian President Volodymyr Zelensky says an alleged Russian agent involved in the deadly attack in Kramatorsk will be charged with treason.

Those helping Russia destroy lives deserve the "maximum penalty", he said.

Twelve people, including three teenagers, were killed in Tuesday's missile strike on a popular restaurant.

Ukraine said the man, a resident of Kramatorsk, sent video footage of the restaurant to the Russian military hours before it was destroyed.

Fourteen-year-old twin sisters Yuliya and Anna Aksenchenko and a 17-year-old girl were among those killed.

"Russian missiles stopped the beating of the hearts of two angels," Kramatorsk city council's education department said in a statement.

At least 60 others were injured, including Colombian nationals and a leading Ukrainian writer.

On Wednesday, Ukrainian security services released a photo of a local man that they arrested, describing him as a Russian agent.

Speaking in his nightly address, Mr Zelensky explained that the country's security services had worked alongside police special forces to detain the suspect - who may face life imprisonment.

Emergency services said on Wednesday that search and rescue efforts were still ongoing.

Kramatorsk, an eastern city in the Donetsk region, is under Ukrainian control but it is close to Russian-occupied parts of the country.

In April last year, more than 50 people were killed and many more were wounded in a missile strike on a train station in the city.

The Ria lounge, which was targeted this time, was a popular venue regularly hosting international journalists, volunteers and Ukrainian soldiers taking a break from the nearby front lines.