Berlin has updated its travel advice for the United States to stress that holding a US visa or entry waiver does not guarantee entry for German citizens.

The change comes after three German nationals were detained while trying to enter the US.

A spokesman for the Foreign Office said the ministry was taking these incidents seriously.

The ministry changed its advisory on Tuesday. It now includes a reminder that approval through the US ESTA system or holding a US visa does not automatically allow entry in every case.

"The final decision on whether a person can enter the US lies with the US border authorities," the spokesperson said Wednesday, noting that the same was true for German authorities.

He added that the change did not amount to a travel warning.

What happened to the Germans detained in the US?

In one of the cases, a German man with a green card residency permit was detained at Boston airport last week after returning from Luxembourg. He has been held in detention since then, according to family members.

In another case, a 25-year-old German was detained while crossing the border from Mexico with his American fiancee in February, according to the Spiegel news magazine. He spent two weeks in detention before being sent back to Germany.

A 29-year-old woman who was also stopped at the US-Mexican border in January was returned to Germany last week, Spiegel reported.

Have other countries changed their advice?

The United Kingdom has also updated its travel advice for citizens heading to the US.

Current information for UK passport holders published online by the British Foreign Office states that travelers "should comply with all entry, visa and other conditions of entry. The authorities in the US set and enforce entry rules strictly. You may be liable to arrest or detention if you break the rules."

The website says the advice was last updated on March 14.

An archived version of the same page from February says only that "authorities in the US set and enforce entry rules."

Earlier this month, news outlets reported that a British woman had been detained for more than 10 days at the US border due to a possible breach of her visa conditions. The Foreign Office later confirmed it was providing support to a British national detained by American authorities.

According to British media, the woman has since returned to the UK.

A shift in US policy

US President Donald Trump has signed a number of executive orders to tighten immigration controls, border security and visa vetting procedures since entering office in January.

The German Foreign Ministry said earlier this week that it was looking into whether the recent detentions were isolated cases or part of a change in American policy.

Tourists from Germany and other EU countries typically have visa-free access to the US for up to 90 days.

Edited by: Louis Oelofse