The United States has urged Russia to launch an immediate investigation into the alleged poisoning of opposition leader and Kremlin critic Aleksei Navalny, who is undergoing treatment in Germany. The US Ambassador to Moscow, John Sullivan, said on Tuesday that the authorities in Russia should open an “immediate, comprehensive, and transparent investigation” that holds the parties behind this act responsible. Mr Navalny, 44, is an outspoken critic of Russian President Vladimir Putin and has exposed official corruption. He fell ill on a flight in Siberia on Thursday before being flown to Berlin on Saturday on a special flight organised by a German NGO. He is currently in a medically induced coma in a Berlin hospital. The ambassador’s calls echoed the European Union’s diplomatic chief Josep Borrell, who on Monday also called on Russia to launch a “transparent” investigation into the apparent poisoning. Mr Borrell added that it was “imperative” that the Russian authorities acted "without delay”. Mr Borrell's statement echoed one from German Chancellor Angela Merkel, who said that those behind Mr Navalny's poisoning "must be held accountable". She, too, called for a thorough and transparent investigation. Earlier on Monday, the Charite Hospital in Berlin said clinical tests performed on Mr Navalny “indicate poisoning with a substance from the group of cholinesterase inhibitors”. Hours later, a senior Russian doctor who was part of the team that treated Mr Navalny in Siberia said they had not detected that toxin in their tests. Cholinesterase inhibitors act by blocking the breakdown of a key chemical in the body, acetycholine, that transmits signals between nerve cells. Mr Navalny is being treated with the antidote atropine. With calls for an investigation mounting, the speaker of Russia's lower house of parliament on Tuesday ordered one of its committees to look into whether foreign forces were behind the alleged poisoning in a bid to fuel tensions in Russia. "The State Duma security committee will be instructed to analyse what happened in order to understand whether this was an attempt on the part of foreign states to harm the health of a Russian citizen to fuel tensions inside Russia, as well as to formulate fresh accusations against our country," speaker Vyacheslav Volodin said in a statement. The Kremlin said earlier on Tuesday that it saw no need to investigate the circumstances leading up to Mr Navalny's grave illness. Kremlin spokesman Dmitry Peskov argued that the German hospital's initial diagnosis of poisoning was not yet conclusive. Mr Navalny's supporters claim that he was poisoned by something in his cup of tea at a Siberian airport.