MOSCOW - Activation of a US-backed missile shield in Romania constitutes a threat to Russia's national security, Kremlin spokesman Dmitry Peskov said on Thursday."Without doubt, the deployment of the PRO system really is a threat to the security of the Russian Federation," Peskov told reporters on a conference call."Measures are being taken to ensure the necessary level of security for Russia. The president himself (Vladimir Putin), let me remind you, has repeatedly asked who the system will work against?"The United States is due to switch on the $800 million missile shield in Romania on Thursday, a step it sees as vital to defend itself and Europe from so-called rogue states.