"We urgently demand that Alexey Navalny receive adequate medical treatment and access to doctors he trusts", Maas told Bild.
Russia's prison authorities, which have barred Mr. Navalny's own medical team from visiting him, said its doctors had made a decision to move him to a medical facility on the premises of another penal colony outside Moscow.
Last month, Biden stepped up his criticism of Russian President Vladimir Putin, calling him a "killer", turning up the volume on increasingly sour relations between the Washington and Moscow.
U.S. President Joe Biden's national security adviser, Jake Sullivan, said Sunday that Moscow had been warned that there will be repercussions if Navalny dies in custody.
"This is super disrespectful to people who came to fulfil their human duty, a medical duty to help a patient", Vasilyeva told AFP outside the colony. "We are talking not just about Navalny's freedom but his life", Navalny's right-hand man Leonid Volkov said on Facebook.
Navalny's lawyers also arrived at the penal colony on Tuesday and were allowed in, an AFP journalist at the scene reported.
European Union figures have been vocal about the issue, with the bloc's foreign ministers releasing a joint statement warning of the risks to Mr Navalny's life on Sunday and meeting to discuss the issue further later on Monday.
The US on Thursday announced sanctions and the expulsion of 10 Russian diplomats in retaliation for what the White House says is the Kremlin's US election interference, a massive cyber attack and other hostile activity.
The United States imposed new sanctions on Russian Federation last Thursday over alleged malign actions, denied by Moscow, including interference in last year's presidential election, cyberhacking, and bullying of neighbouring Ukraine.
In recent years, Mr. Navalny, through brilliant combination of investigative technique, video production and social media savvy, became the leading voice criticizing Mr. Putin, documenting how the onetime KGB officer in the Kremlin became mega-rich.
Navalny launched his hunger strike demanding proper medical treatment over severe back pain and numbness in his limbs.
Navalny's physician, Dr. Yaroslav Ashikhmin, said Saturday that test results he received from Navalny's family show him with sharply elevated levels of potassium, which can bring on cardiac arrest, and heightened creatinine levels that indicate impaired kidneys.
Navalny's team has called on supporters to take to the streets on Wednesday - when Putin is due to deliver his annual state of the nation address - to protest how he is being treated.
The anti-corruption campaigner's allies, who routinely face police searches and arrests for their activism, are facing increasing pressure from the authorities ahead of the rallies.
Ivan Zhdanov from Mr Navalny's team said on Monday that the official statement did nothing to assuage their worst fears.