The poisoning of Kremlin critic Alexei Navalny “constitutes a threat to international peace and security,” Britain, France, Germany, Estonia and Belgium wrote in a letter to the United Nations Security Council, seen by Reuters on Thursday.

We call on the Russian Federation to disclose, urgently, fully and in a transparent manner, the circumstances of this attack and to inform the Security Council in this regard,” they said in the letter sent to the 15-member body late on Wednesday.

Russian U.N. Ambassador Vassily Nebenzia told reporters that Russian authorities were still waiting for Germany and France to respond to their request for evidence so Moscow could assertain whether they needed to start an investigation.

Navalny was flown to Berlin in August after falling ill on a Russian domestic flight. He received treatment for what Germany said was poisoning by a potentially deadly nerve agent, Novichok, before being discharged in September.

The letter to the Security Council said that on Sept. 2 the German government confirmed that tests had shown “unequivocal proof that Mr Navalny had been poisoned by a chemical nerve agent from the Novichok group, which was developed by the Soviet Union and subsequently held by its successor state” Russia.

Russia has denied any involvement in the incident and said it has yet to see evidence of a crime. The Russian mission to the United Nations did not have an immediate comment on the letter.

The European members of the Security Council and Britain noted that last November the Security Council adopted a statement reaffirming that any use of chemical weapons “anywhere, at any time, by anyone, under any circumstance is unacceptable and a threat to international peace and security.”

“As such, we consider that the use of a chemical nerve agent from the Novichok group in the abhorrent poisoning of Mr Alexey Navalny constitutes a threat to international peace and security,” they wrote.

The letter was sent as Russia takes the monthly presidency of the Security Council for October.

The United States did not sign the letter, but on Wednesday U.S. Secretary of State Mike Pompeo said: “The Russian Government must provide a full accounting for the poisoning of Alexei Navalny and hold those involved responsible.”

Reuters