German Defence Minister Boris Pistorius said the federal government is considering withholding the information it shares with state ministers if governments led by the far-right Alternative for Germany (AfD) take office.
His comments come as opinion polls suggest the AfD could secure an outright majority in September's state election in Saxony-Anhalt, potentially allowing the party to lead a state government for the first time.
Under Germany's federal system, state governments hold extensive powers in areas including policing and intelligence services, giving them access to sensitive government information.
Pistorius said authorities are closely reviewing who can access classified information, amid questions over whether future state governments led by the AfD could affect military bases.
"We are obliged to do so because it is about the security of our country," Pistorius told Germany's Bild newspaper.
The centre-left Social Democrat (SPD) said he would be uncomfortable sharing classified information with an AfD state minister, citing what he described as the party's close ties to Russian President Vladimir Putin.
"You only have to listen to the public statements of many, many AfD representatives. Their proximity to Putin is impossible to overlook," he said.
The AfD has long faced criticism over its perceived links to Kremlin.
Another state in the AfD's eastern stronghold, Mecklenburg-Western Pomerania, is also set to hold elections in September.
The party is leading in opinion polls there as well, although it is considered less likely to win an absolute majority.
Since finishing second in last year's general election, the AfD has steadily gained support nationwide, while Chancellor Friedrich Merz's centre-right CDU/CSU alliance has slipped into second place.
A national poll published on Sunday put the AfD at 29 percent, giving it an 8-point lead over the CDU/CSU, which stood at 21 percent.
