Ukraine's Volodymyr Zelensky proposed a face-to-face meeting with Vladimir Putin in a rare open letter to the Russian leader Thursday, shortly after the Kremlin chief had conceded Moscow needed to strengthen its air defences following a spate of Ukrainian attacks. The Kremlin said Putin had not yet been shown the letter, but that Zelensky could meet Putin in Moscow "any time" -- a proposal that the Ukrainian leader preemptively ruled out in his letter.