Serbian tennis star Novak Djokovic has offered help to retired Ukrainian player Sergiy Stakhovsky, who has returned to his country to fight against Russia.

Stakhovsky, who won four ATP titles, posted on Instagram Sunday a screenshot of his WhatsApp conversation with the Serbian superstar.

"Thinking of you ... hope all calms down soon," Djokovic said.

"Please let me know what would be the best address to send help. Financial help, any other help as well ... "

To the messages by the Serb, Stakhovsky replied: "Nole thanks a lot, I am on the ground Kyiv is pretty silent."

Stakhovsky, 36, who announced his retirement in January, has left his wife and children in Hungary to sign up for Ukraine's military reserves.

In another post in which he shared a photo of him with his family, Stakhovsky said: "We are all fighting for our children's future ... Wait a little bit longer ... I will return to you."

Earlier, he said he never imagined he will have to wear a bulletproof vest in Kyiv. "It's a disaster the way Russia invaded Ukraine ... Bombing cities ... killing innocent people ... World we must unite to make it stop ..." he said.

The war on Ukraine, which began on Feb. 24, has drawn international condemnation, led to financial sanctions on Moscow and spurred an exodus of global companies from Russia.

More than 1.7 million people have fled Ukraine to neighboring countries, according to the UN Refugee Agency.