Billionaire Elon Musk defied Russian President Vladimir Putin on Monday to fight him in a "man-to-man fight" over Ukraine, which has been under attack by the Russian army for nearly three weeks.

"I challenge Vladimir Putin to do a man-to-man fight over Ukraine," the SpaceX founder wrote on Twitter, without specifying what form the duel would take.

"Do you accept this fight?" Asked in another message in Russian, in Russian, to the official Kremlin account on Twitter, "Do you accept this fight?"

The South African-born billionaire had supported Kyiv shortly after the start of the Russian war on it, and earlier this month he tweeted, "Hold on, Ukraine," at the same time, sending a "message of friendship to the great Russian people who do not want" war.

He also responded to a request for help from a Ukrainian official by activating the satellite Internet service "Starlink" in Ukraine, and sending equipment there to help provide Internet service to areas that have been subjected to Russian military attacks.