Greece has rejected pressure from European allies to help Ukraine boost its air defences, arguing that Athens needs the systems for itself.
Ukraine's Volodymyr Zelensky has appealed for at least seven more Patriots or similar defence batteries to protect its cities and power plants from Russian attack.
But Greece has said it cannot spare any of its Patriot or S-300 systems.
Reports say Spain will supply some Patriot missiles but not a full system.
Spain and Greece have come under pressure to help Ukraine from both Nato and European Union allies, in response to attacks on vulnerable cities including Chernihiv and power facilities in Kharkiv and close to Kyiv.