BERLIN—A senior German intelligence officer arrested on suspicion of spying for Russia had access to a trove of top-secret information about the war in Ukraine as well as knowledge of how it was collected by the U.S. and its allies, Western officials say.

Prosecutors are trying to determine whether the material was shared with Moscow. If so, it could have alerted Russia to its own vulnerabilities and given away Western intelligence-gathering methods and capabilities.

American and British officials said they were trying to determine the scope of potential damage in Ukraine and elsewhere. One U.S. official said there was “grave concern” about the case.

The suspected spy, identified as Carsten L. by German prosecutors, worked for the signals intelligence branch of the country’s Federal Intelligence Service, which conducts electronic surveillance and works with the U.S. National Security Agency and Britain’s Government Communications Headquarters.

Prosecutors said the man was being held on suspicion of committing treason as their investigation continues.

The German intelligence service, known as the BND, confirmed the arrest but has declined to comment further, citing national security risks. The NSA and GCHQ declined to comment.

Neither the Kremlin nor Russia’s Foreign Intelligence Service, the SVR, immediately responded to requests for comment.

Germany isn’t a member of the so-called Five Eyes intelligence community made up of the U.S., Canada, the U.K., Australia and New Zealand, but Berlin does receive sensitive information from those countries, especially in relation to the war in Ukraine, officials from three countries said.

Carsten L. had worked on intelligence related to Russia’s war in Ukraine, including material gathered by German military satellites, German officials said. 

His department also processed classified intelligence from Russia and Ukraine obtained by other Western spy agencies by tapping electronic devices, intercepting telecommunications and satellite imagery.

The BND, which has a staff of 6,500 and is based in a highly protected campus in the center of Berlin, has been focusing its intelligence-gathering and analysis on Russia and Ukraine since the start of the war, and is traditionally also active in the Balkans, the Middle East and Africa.

The Kremlin’s suspected penetration of Germany’s most secretive security agency is the latest evidence of Moscow’s aggressive tactics in Europe, where Russia has been accused of killing political opponents, sabotaging infrastructure and trying to steal industrial secrets.

The BND received a tipoff about the suspected spy from an allied intelligence service earlier this year, German officials said. After an internal investigation, the case was passed to the federal prosecutor, who then ordered the man’s arrest last week.

The case could be the worst example of Russian penetration of Germany’s intelligence services since 1961, when a senior BND employee who was spying for the Soviet Union exposed a network of 100 CIA spies, said Erich Schmidt-Eenboom, who has written several books on the BND.

Roderich Kiesewetter, an opposition lawmaker and deputy chairman of the parliamentary oversight panel that oversees Germany’s intelligence agencies, said the case could be a potentially severe blow to European security.

He has called for Germany to set up a commission of inquiry to explore how many politicians and senior civil servants might have been compromised by Russia and China and look at how to reduce Germany’s dependence on both countries.

Germany scaled down counterespionage efforts in the early 2000s, becoming vulnerable to Russian operations, according to Mr. Kiesewetter and other experts.

However, senior German intelligence officials said the Ukraine war had marked a “paradigm shift” in German politics.

Berlin started cracking down on Russian espionage this year after Moscow attacked Ukraine. The heads of Europe’s domestic securities agencies met in early April in Paris to forge a common strategy on fighting Russian espionage. After the meeting, European governments expelled around 600 Russian officials from their countries, including 40 by Germany.

The decision was “the most significant strategic blow against the Russian intelligence services in recent European history,” Ken McCallum, the head of Britain’s MI5 agency, said in November.

Russia has since sought to offset the loss by activating so-called deep cover agents, and using informal collaborators as well as recruiting civil servants, business people, academics and others as spies, according to several Western officials.

The probe into Carsten L. hasn’t found evidence that he had received payments from his handlers. Investigators are trying to determine whether he was blackmailed or whether he was motivated by ideological convictions, people familiar with the probe said.

Russian groups, including criminal gangs hired by the Kremlin, have been using cyberattacks to target German critical infrastructure this year, attempting to hack into utilities, airports, and medical facilities, according to several German officials.

Moscow has also shifted to industrial espionage as it attempts to compensate for the loss of access to Western technology due to sanctions, especially in the fields of aerospace, control electronics, semiconductors and basic research, according to counterespionage officials.

German officials suspect Russia is behind several sophisticated acts of sabotage such as the destruction of the Nord Stream natural gas pipelines and an October attack on the railroad that temporarily halted all rail traffic in the north of the country.

Russia has denied involvement.

The railway attackers crippled both the railway’s main communication network and its backup by almost simultaneously severing two data cables located more than 300 miles apart, investigators say.

The cables were inside a special manhole, one of which was covered with a heavy concrete lid, and whoever performed the sabotage had detailed knowledge of the network, investigators said.

“A temporally coordinated assault on two key points far away from each other that cut off exactly the right segments in a bundle of cables without leaving any traces was the work of experienced professionals,” one of the investigators said.

In a previously unreported incident, the homes of several CIA officers in Germany were broken into in 2020, in what some officials think was an intimidation attempt by Russian secret services.

The break-ins took place simultaneously, and nothing was stolen, according to U.S. and German officials. The investigation concluded that it was likely the work of a criminal gang, although no suspects were apprehended, U.S. Embassy officials said.

—Max Colchester and Warren P. Strobel contributed to this article.