MOSCOW (Reuters) - The European Union's ambassador to Russia said on Tuesday he was appalled by "disgusting" murder threats made by Chechnya's leader Ramzan Kadyrov to Mikhail Kasyanov, a Russian opposition figure, and urged the authorities to react immediately.

Kadyrov, one of Russian President Vladimir Putin's most loyal allies, stirred fear among Kremlin critics on Monday by releasing a video showing opposition leader Kasyanov in the cross-hairs of a sniper's rifle.

"EU Ambassador to Russia (Vygaudas) Ušackas considers that the recent statements by Kadyrov towards the Russian political opposition and civil society are unacceptable in the democratic state," the EU mission in Moscow said in a statement after a telephone conversation between Ušackas and Kasyanov.