A Russian plot to kill one of Europe’s most prominent defense-industry executives signals a significant escalation in Moscow’s covert efforts to sabotage Western weapons production and weaken support for Ukraine, Western officials said.

Earlier this year, the U.S. warned Germany that Moscow had set in motion a plan targeting Armin Papperger , CEO of Rheinmetall , a German maker of tanks, armored vehicles and ammunition, which has been integral to Western efforts to support Ukraine as it fights to fend off invading Russian forces, U.S. and German officials said.

There was an “intensified threat situation,” especially at the beginning of this year, one person familiar with the matter said. German authorities warned Papperger and ramped up his security, and Russia didn’t follow through with the planned attack, the person said.

A German Interior Ministry spokesman said “the government takes the threat from the Russian regime very seriously.” He said Moscow was waging a campaign of threats, cyberattacks, disinformation, sabotage and at least one act of terrorism in Germany to weaken Berlin’s support for Kyiv.

The spokesman declined to elaborate on the alleged terrorist act.

The Kremlin didn’t immediately respond to a request for comment. Kremlin spokesman Dmitry Peskov told journalists on Friday “it is very difficult for us to comment on media reports that contain no serious argumentation and are based on anonymous sources.”

CNN first reported Western officials’ allegations about a plot on Papperger’s life on Thursday.

Russia has been engaged in an intensifying hybrid campaign targeting arms makers, politicians and others across Europe with the aim of undermining military and financial support for Kyiv, Western officials say.

Germany has increased security around Papperger since the start of the year, according to a person familiar with the matter. The CEO now travels with bodyguards, including when attending football matches, while armed police guard the company’s offices when he is there, this person said.

Other Western arms makers have supplied Kyiv with weapons, and several now have repair operations in Ukraine. But Papperger has been more outspoken about his company’s plans, with the CEO making frequent statements about his intentions to make weapons there.

In May, arsonists burned down a garden shed at one of Papperger’s houses. While police have a letter claiming responsibility from a radical left-wing group, investigators said they were also looking at whether Russia could be behind it.

Rheinmetall says hundreds of its vehicles and millions of rounds of its ammunition have been sent to Ukraine. In June it opened a repair center in the country, and Papperger has said the company plans to set up plants in Ukraine to assemble armored vehicles and produce tanks and artillery shells.

Papperger met Ukrainian President Volodymyr Zelensky last year. undefined undefined Russia has ordered at least one killing in Germany before, although it didn’t target a German national. In 2021, a German court convicted a Russian hit man of shooting dead a former Chechen rebel in a Berlin park on orders from the Kremlin.

The killer, Vadim Krasikov , is serving a life sentence in Germany. Russian President Vladimir Putin has signaled an interest in swapping him for Evan Gershkovich , The Wall Street Journal reporter currently held in Russia and falsely accused of espionage.

Among the recent acts of alleged sabotage that Western security officials attribute to Russia are a fire that severely damaged a Berlin plant belonging to a defense contractor in May. The previous month, Germany detained two dual German-Russian nationals on suspicion of spying for Moscow with the goal of preparing attacks on military installations in the country, including a U.S. military base and training area in Grafenwöhr, southern Germany.

“Russia is carrying out a hybrid war of aggression. As Europeans, we must protect ourselves as best possible and we must not be naive,” German Foreign Minister Annalena Baerbock said on Thursday in Washington when asked about the plot against Papperger.

Asked about Russia’s campaign of intimidation, Danish Prime Minister Mette Frederiksen told an event on the margins of this week’s NATO summit in Washington: “I think they are attacking us every day now. Not only on critical infrastructure: Hybrid attacks, cyberattacks, disinformation.”

European arms manufacturers have been an integral part of Western efforts to back Kyiv. Britain’s BAE Systems has a repair facility in Ukraine and intends to eventually make light howitzers in the country, while Babcock International , another U.K. defense company, is setting up a maintenance depot there.

Other companies, including those in the U.S., are supplying weapons, typically through donations from their governments.

Security has always been an issue for defense companies. Security expenses for U.S. defense executives are routinely charted in corporate documents. A spokesman for Sweden’s Saab, whose antitank and other weapons are in Ukraine, said the company already has “good preparedness and routines to protect the business and our employees.”

Write to Bojan Pancevski at bojan.pancevski@wsj.com and Alistair MacDonald at Alistair.Macdonald@wsj.com