The French authorities said the academic had been traveling to a conference near Houston when border officials blocked his entry because of conversations found on his phone.

A French scientist has been denied entry into the United States, apparently because the scientist had expressed a personal opinion on the Trump administration's research policy, the French Education Ministry said on Thursday.

Several European countries, including the UK, Germany, and France, have issued or updated travel warnings to the U.S. due to concerns about how international travelers are treated at entry points.

The United Kingdom updated its advice for travel to the United States on Thursday, warning of harsh consequences for British passport holders who violate U.S. immigration laws. The notice comes just a day after Germany updated its travel advice after three of its citizens were detained trying to enter the U.