Germany warned Russia Wednesday of "unavoidable" targeted sanctions if it failed to shed light on the poisoning of Alexei Navalny, as the Kremlin critic urged the EU to come down hard on Vladimir Putin's inner circle.

The stern message came a day after the UN's chemical weapons watchdog OPCW confirmed Germany, France and Sweden's finding that the Russian opposition leader was poisoned by a nerve agent of the Soviet-era Novichok group.

"A serious violation of international law was perpetrated with a chemical warfare agent, and something like that cannot remain without consequences," German Foreign Minister Heiko Maas told lawmakers.

"It is clear that if the events are not cleared up, if the necessary information is not provided, then targeted and proportionate sanctions against those responsible on the Russian side will be unavoidable."

Navalny was medically evacuated to Germany in late August after falling ill on a plane and spending several days in a Siberian hospital.

He was discharged after just over a month in the Berlin Charite hospital and has vowed to return to Russia to resume his opposition campaign when he is fully recovered.

Western powers have demanded answers from Moscow. Navalny himself has accused Russian President Vladimir Putin of being behind the poisoning, noting that only top officials can deploy the military grade nerve agent.

But Russia has firmly rejected the allegations, lashing out on Tuesday at a "conspiratorial scenario" planned in advance.

The case has plunged Russia's relations with Germany to a new low, as it comes just a year after a murder in a central Berlin park that German prosecutors say was ordered by Moscow.

A Russian suspect is standing trial over the murder, with the court hearing opening Wednesday.

The brazen murder in the heart of the German capital appeared to be a tipping point for Chancellor Angela Merkel, who said in May that the killing "disrupts a cooperation of trust" between Berlin and Moscow.

The German leader has always stressed the importance of keeping dialogue open with Putin, but she has toughened up her tone in recent months. 

- Oligarchs and top officials -

With tensions running high, calls had grown on Berlin to scrap a controversial 10-billion-euro ($12 billion) pipeline project, which is set to double Russian natural gas shipments to Germany. 

Asked during the parliamentary question-time if the Nord Stream 2 pipeline project could be hit, Maas suggested that any embargo would be limited to individuals.

"We have always said that all options are on the table, but we want to agree on the reaction with the EU," he said.

He noted that the pipeline project also involved other countries including Austria and the Netherlands and well over 100 European companies.

"In our opinion, after the discussions we had, it is most likely for the EU to find agreement on a list of people" on whom sanctions would be imposed, he said.

The apparent plan for targeted embargoes dovetailed with Navalny's call.

The 44-year-old Kremlin critic, who is recovering in Berlin, urged the European Union to take action including entry bans against key figures in or backing Putin's government.

In an interview with Bild daily, the Kremlin critic took aim directly at Valery Gergiev, chief conductor at the Munich Philharmonic, citing the musician as someone who should be held to account for his unapologetic backing of Putin.

"He is the perfect example. Such people must be put under pressure," said Navalny.

"People like him must be slapped with entry bans and you know what? 99 percent of Russians would welcome that."

Navalny stressed that any embargo must not hurt the general Russian population. 

Rather, "the most important is to impose entry bans against those profiting from the regime and to freeze their assets," said Navalny.

"Oligarchs and high-ranking officials, Putin's closest circles," he said.