Russian opposition politician Alexei Navalny was poisoned with a Novichok nerve agent, Germany's government says.

It said toxicology tests at a military laboratory showed ""unequivocal proof" of an agent from the Novichok group.

Mr Navalny was airlifted to Berlin for treatment after falling ill during a flight in Russia's Siberia region last month.

He has been in a coma since.His team says he was poisoned on President Vladimir Putin's orders.

The Kremlin has dismissed the allegation. The German government said it condemned the attack in the strongest terms and called for Russia urgently to provide an explanation.

"It is a disturbing development that Alexei Navalny was the victim of a chemical nerve agent in Russia," it said.

Chancellor Angela Merkel has met senior ministers to discuss the next steps, the statement said.

The Kremlin said it had not received any information from Germany that Mr Navalny had been poisoned using a Novichok nerve agent, Russia's Tass news agency reported.

The German government said it would inform the EU and Nato of its findings."[The federal government] will discuss an appropriate joint response with the partners in the light of the Russian response," it said.

Mr Navalny's wife Yulia Navalnaya and Russia's ambassador to Germany would also be informed of the findings, the statement said.

What is Novichok?

The name Novichok means "newcomer" in Russian, and applies to a group of advanced nerve agents developed by the Soviet Union in the 1970s and 1980s.Novichok agents have similar effects to other nerve agents - they act by blocking messages from the nerves to the muscles, causing a collapse of many bodily functions.

While some Novichok agents are liquids, others are thought to exist in solid form. This means they could be dispersed as an ultra-fine powder.

Novichoks were designed to be more toxic than other chemical weapons, so some versions would begin to take effect rapidly - in the order of 30 seconds to two minutes.