"The conclusion that the murder was authorized at the highest levels of the Russian state is extremely disturbing," the spokeswoman told reporters. "It is not the way for any state, let alone a permanent member of the United Nations Security Council, to behave."
"In light of the inquiry's findings we are considering what further actions we should take," she said.
An inquiry led by British judge Robert Owen found that former KGB bodyguard Andrei Lugovoy and another Russian, Dmitry Kovtun, poisoned Litvinenko as part of an operation directed by Russia's Federal Security Service (FSB), the main heir to the Soviet-era KGB.
"The FSB operation to kill Mr Litvinenko was probably approved by Mr Patrushev, then head of the FSB, and also by President Putin," Owen said.