The German foreign ministry has updated its travel advice for Germans travelling to the US after three German nationals were denied entry and detained as they tried to enter.

"A criminal conviction in the United States, false information regarding the purpose of stay, or even a slight overstay of the visa upon entry or exit can lead to arrest, detention, and deportation upon entry or exit," information on the ministry's website now explicitly says.

Advice for travellers now also warns that possessing an electronic system for travel authorisation (ESTA) document – the automated system that determines the eligibility of visitors to travel to the US under the visa waiver programme – does not automatically guarantee entry into the US.

"The final decision on whether a person can enter the United States rests with the American border authorities. But this is no surprise; it is the same in Germany," a ministry spokesperson told German daily Der Spiegel.

The foreign ministry was also keen to point out that the updated guidance doesn't constitute a travel warning to the US.

The updated travel advice comes after the foreign ministry said on Monday it was probing the case of three of its citizens who had been denied entry and placed in detention as they tried to enter the US.

"The Federal Foreign Office is aware of three cases in which German citizens were unable to enter the United States and were placed in deportation detention upon entry," a foreign ministry spokesperson said.

Among those detained was Fabian Schmidt, 34, a legal permanent US resident. According to US outlet WGBH, he was detained at an airport in Boston before being transferred to a detention facility in Rhode Island.

Schmidt's mother, Astrid Senior, claimed in an interview that her son was "violently interrogated" at the airport before being stripped naked and forced into a cold shower by two officials.

The two other nationals affected were Jessica Brösche, a 29-year-old tattoo artist from Berlin, and Lucas Sielaff, 25, from Saxony-Anhalt. Both have since been sent back to Germany.

Brösche had attempted to enter the US from Tijuana in Mexico while travelling with her friend, a US citizen.

According to the online fundraiser set up to fund her return, authorities originally told her she would be detained for several days, but that what ensued instead was an "alarming sequence of events" with Brösche transferred and kept at the Otay Mesa Detention centre for more than six weeks.

Brösche's friends alleged she was put in solitary confinement for nine days during her ordeal.

According to ABC 10News, San Diego CoreCivic, the company that owns the detention centre where Brösche was held, denied those claims.

Sielaff returned to Germany in early March after two weeks in detention his girlfriend, Lennon Tyler, told Swiss daily Tages-Anzeiger. He had entered the US on a tourist visa and visited Mexico for a short trip.

Germans who have a valid tourist visa to the US are generally allowed to travel visa-free for up to 90 days, according to the US embassy website in Germany.

Green card holders are generally allowed to travel abroad and re-enter the US after stays lasting less than six consecutive months, according to the US government.