BERLIN (REUTERS) - The novichok nerve agent used to poison Kremlin critic Alexei Navalny was "harder" than previous forms, Der Spiegel magazine reported the head of Germany's foreign intelligence service as saying.

Bruno Kahl, head of Germany's foreign intelligence service, informed what Spiegel termed a "secret meeting" about the potency of the poison, the magazine said, but did not give any other details of the meeting.

Spiegel added that a delegation from the Organisation for the Prohibition of Chemical Weapons (OPCW) was said to have visited at the weekend the Berlin hospital where Navalny is being treated.

Navalny, President Vladimir Putin's highest-profile critic, was airlifted to Germany last month after falling violently ill while travelling in Siberia.

German doctors say he was poisoned with novichok, a Russian nerve agent.

Moscow says it has seen no evidence he was poisoned.

Der Spiegel reported on Thursday that Navalny had made further progress in his recovery, and was able to speak again.