Germany's foreign office has updated its official advice and information about travel to the US.
A spokesperson for the foreign office told German media that "We now clearly emphasise that an ESTA authorisation or a US visa does not entitle you to enter the US in every case."
Ultimately the decision on whether or not a person may enter the US "lies with the US border authorities," they added.
Similarly, whether or not people can enter Germany is ultimately up to the German border authorities.
Spiegel reported that the foreign office has warned potential travellers to the US coming from Germany that criminal records, false information about the purpose of stay or even a slight overstay when traveling can lead to arrest, and deportation detention.
The change comes following three cases where German nationals were denied entry to the US and detained for days or weeks despite having visas or residence permits for the US.
In the most recent case, Fabian Schmidt, a German man with a green card residence permit for the US was detained at Boston airport last week after returning from Luxembourg.
He has been held in detention since then, according to family members who added that his residence permit had just been renewed and there were no ongoing legal proceedings against him.
According to US media reports, customs agents had stripped Schmidt, put him in a cold shower, and held him in a bright room with little food or water, where he was denied access to his medication and suffered sleep deprivation.
Schmidt's mother also claims immigration officers had pressured him to give up his green card.
In response to those incidents, Germany had previously said it was "monitoring the situation" and would "if necessary, adjust its travel and security advice."
On Tuesday, the foreign office decided to adjust the travel and security advice especially regarding entry controls.
However a foreign office spokesperson has emphasised that the change in policy did not amount to a travel warning.
What documents are required to enter the US?
Tourists from Germany and other EU countries typically have had visa-free access to the US for up to 90 days.
According to information from the foreign office, Germans need a passport as well as a a valid ESTA (Electronic System for Travel Authorization) document to enter the US.
The same applies to other non-US nationals who travel to the US from Germany. Although some foreign nationals would also need a visa, depending on their nationality.
It appears that following executive orders by US President Trump, border security measures have been tightened.
The German Foreign Ministry said earlier this week that it was looking into whether the recent detentions were isolated cases or part of a more serious change in American policy.
US tech media outlet The Verge reported that the arrests of people who legally should have been allowed to enter the US appears to be due to the misuse of a surveillance system deployed by the US Department of Homeland Security.
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