BERLIN (AP) — German Chancellor Angela Merkel faced pressure on Thursday to use a joint German-Russian pipeline project as leverage in getting Russia to provide answers about the poisoning of the Kremlin's most determined critic.

After German authorities said Wednesday that tests showed Russian opposition leader Alexei Navalny was poisoned with a Soviet-era nerve agent, Merkel called the attack a murder attempt and demanded Moscow's cooperation.

“There are very serious questions now that only the Russian government can answer, and must answer,” Merkel said.

The chancellor said Germany would work with the European Union and NATO allies on a joint response. The opposition Green party said that should include an end to the Nord Stream 2 pipeline project, which would deliver Russian gas directly to Germany under the Baltic when completed.

“The apparent attempted murder by the mafia-like structures of the Kremlin can no longer just give us cause for concern, it must have real consequences,” Green parliamentary group leader Katrin Goering-Eckardt said.

The Greens have long opposed the pipeline. The project with Russia also divides opinion in other camps. Norbert Roettgen, a lawmaker in Merkel's party who chairs parliament's foreign affairs committee, said that “diplomatic rituals are no longer enough.”

“After the poisoning ... we need a strong European answer, which #Putin understands” Roettgen tweeted. “The EU should jointly decide to stop #NordStream2.”

In Moscow, Kremlin spokesman Dmitry Peskov said calls to abandon Nord Stream 2 were “emotional statements... not based on facts.” He called the pipeline “an international commercial project that is in the interests of Russia, Germany and the entire European continent.”

Peskov once again brushed off allegations of Russian government’s involvement in Navalny's poisoning and urged German authorities to provide evidence to Russia.

“We haven’t received any information so far. We hope that it will happen soon and will help figure out what caused the condition the patient is in right now,” Peskov said.

Navalny fell ill on a flight to Moscow on Aug. 20 and was taken to a hospital in the Siberian city of Omsk after the plane made an emergency landing.

He was moved two days later to Berlin’s Charite hospital, where doctors last week said initial tests indicated Navalny had been poisoned.

On Wednesday, the German government said that testing by a German military laboratory showed “proof without doubt of a chemical nerve agent from the Novichok group.” British authorities identified Novichok as the poison used on former spy Sergei Skripal and his daughter in England.

Navalny remains in an induced coma in the hospital. Doctors said that while his condition was improving, the politician and anti-corruption investigator faces a long recovery and long-term effects cannot be ruled out.

Despite Merkel's strong reaction to the news, it was not clear whether she would consider trying to put an end to the pipeline project.

After doctors in Berlin reported last week that Navalny had likely been poisoned, Merkel rejected the idea of abandoning the project. Merkel said the pipeline should be completed and that she did not “think it is appropriate to link this business-operated project with the Navalny question.”

Merkel also rejected the idea when it was floated after previous confrontations with Moscow over incidents closer to home - like evidence the chancellor's parliamentary office had been hacked by Russia and the killing of a Georgian man in downtown Berlin that prosecutors alleged was a hit ordered by the Russian state.

Merkel also steadily resisted pressure from the United States to end the project, which Washington says will endanger European security by making Germany overly dependent on Russian gas. Nord Stream 2 is also opposed by Ukraine and Poland, which will be bypassed by the pipeline under the Baltic sea, as well as some other European nations.

In addition to the security concerns, the U.S. also wants to sell more of its own liquefied natural gas, or LNG, to Germany, which has Europe’s largest economy.

Last month, three Republican U.S. senators threatened sanctions against a German port operator involved in the project, prompting Germany's foreign minister to bring up the issue with Secretary of State Mike Pompeo.

Switzerland-based Allseas, which operates ships laying sections of the undersea pipeline, suspended its work in December after U.S. President Donald Trump signed legislation threatening sanctions against companies linked to the project.

Despite the calls from some to bring Nord Stream 2 into the Navalny case, others have spoken against such a move. Wolfgang Kubicki, deputy leader of the pro-business Free Democratic Party, warned that Nord Stream 2 is nearing completion and both Russian and German companies are massively invested in the project.

“I'm skeptical that we should question a project of this magnitude at this stage,” he told Deutschlandfunk radio.

And Markus Soeder, a Merkel ally who leads the smallest party in Germany's governing coalition, said the construction of the pipeline was a private business decision, not the government's.

"In our view, one has nothing to do with the other," he said.

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Daria Litvinova in Moscow contributed to this story.