Russian Foreign Minister Sergei Lavrov said that Moscow would only cooperate with the U.K.’s investigation into the nerve-agent poisoning of a former spy if London supplied the substance in question and opened up the probe to Russian officials.

As ties between the two countries hit their lowest point since the Cold War, Russia is facing an end-of-Tuesday deadline set by U.K. Prime Minister Theresa May to offer an explanation for the nerve-agent poisoning of Sergei Skripal and his daughter Yulia or face possible retaliation.