One of the U.K.’s top spies has made a rare public speech, accusing Russia of “flagrant breaches of international rules” over a poisoning attack on U.K. soil in March.

In a speech to security chiefs in Berlin Monday, MI5 Director-General Andrew Parker said the “reckless attempted assassination of Sergei Skripal, using a highly lethal nerve agent, put numerous lives at risk, including that of his daughter.”

It was the first time in history that the boss of Britain’s domestic spy agency spoke publicly at a venue outside of the United Kingdom. Parker also said there was a need to “shine a light through the fog of lies, half-truths, and obfuscations that pours out of their propaganda machine.”

It is now 10 weeks since a nerve agent attack on the former Russian spy Skripal and his daughter Yulia, in the English city of Salisbury. The pair were found unconscious on March 4 on a bench.

Skripal remains in hospital but is no longer in a critical condition, while his daughter was discharged last month. No suspect has been identified but the United States and other European countries joined with Britain in blaming Russia for the attack.

Moscow has denied responsibility. It has also retaliated against a co-ordinated move by Western countries to expel 150 Russian diplomats by ordering out the same number of opposing officials from foreign embassies in Russia.

Professor of International Law at Lancaster University, James Sweeney, told CNBC Monday that it was becoming clearer that the U.K. is convinced Russia was responsible for the Salisbury poisonings. He said if the U.K. is both able and willing to provide more evidence, it could pave the way for establishing to what extent Russia has broken international law.

“It could be seen as a crime against humanity and therefore the International Criminal Court could, in theory, try to hold (Russian President Vladimir) Putin himself responsible,” Sweeney said, before adding that without Russian state cooperation this avenue would be difficult to enforce.

Sweeney said other options included seeking a rebuke from the United Nations should the poisoning be found as a use of force in international law. While such an action is prohibited in the UN charter, Sweeney noted that Russia’s position as a permanent member of the UN Security Council would prove a major obstacle.

He said a third possibility could be if the U.K. raised the issue at the International Court of Justice which can require an aggressor state to pay compensation.