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Several U.S. universities have issued advisories to students in the wake of President Donald Trump’s executive order temporarily restricting entry from seven predominantly-Muslim nations, with some institutions advising against international travel until its effects are more understood.

The order suspends entry for 90 days from certain nations based on statute related to the Visa Waiver Program. The most recent version of that program named Iraq, Syria, Iran, Sudan, Libya, Somalia, and Yemen. The order contains some visa exemptions.

Related: Trump Travel Restrictions Leave Refugees Stranded: Reports

"At a practical level, we are advising community members and visiting scholars from the designated countries to suspend plans for international travel," Columbia University President Lee C. Bollinger said in a statement released Sunday, adding that they did not know of any Columbia students, faculty or staff who had been affected.

"We join with many peers in decrying this action as discriminatory, damaging to America’s leadership in higher education, and contrary to our nation’s core values and founding principles," he added.

Stanford's Bechtel International Center listed in a Facebook post Friday the seven countries that could be affected and said "we recommend that nationals of these countries do not travel."

A Stanford PhD student flying into New York late Friday night from Sudan was detained for around five hours after Trump signed the order, university spokesperson Lisa Lapin said. The student, who is from Sudan but is a longtime legal U.S. resident with a green card, was released at 4:30 a.m.

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"We have strongly advised students and scholars who might be affected and who have travel plans in the coming days to defer travel outside of the United States until there is some clarity and legal analysis of the situation or, if they must travel, to seek legal counsel before they do," Princeton University said in an email.

The Massachusetts Institute of Technology said in a statement to students Saturday that the order “is already having an impact on members of our community."

"While we are very troubled by this situation, our first concern is for those of our international students and scholars who are directly affected. We are working closely with them to offer every support we can,” the letter from Provost Martin A. Schmidt, Chancellor Cynthia Barnhart, and Vice President for Research Maria T. Zuber said.

The University of Notre Dame also urged the president to rescind the order. The executive order "will demean our nation, whose true greatness has been its guiding ideals of fairness, welcome to immigrants, compassion for refugees, respect for religious faith and the courageous refusal to compromise its principles in the face of threats," the university said in a statement released Sunday.

Trump signed the order as part of what he called an effort to prevent foreign terrorists from entering the U.S.

It also barred entry by all refugees for 120 days, and suspended indefinitely admittance of Syrian refugees until the president is satisfied with changes made to the U.S. Refugee Admissions Program.

The order allows some exemptions, and it is unclear how many students might be affected.

Statistics maintained by the Department of Homeland Security indicate about 17,000 students from the seven designated countries were allowed into the U.S. for the 2015-2016 school year, The Associated Press reported.

Samira Asgari, an Iranian doctor, tweeted that she intended to travel to Harvard Medical School’s Brigham and Women's Hospital but was denied boarding. She told NBC Boston that she wanted to come to the U.S. "to learn, to be able to go forward in my career."

I was pretty excited to join @soumya_boston's lab but denied boarding due to my Iranian nationality. Feeling safer? — Samira Asgari (@samsam_86) January 28, 2017

The University of Michigan said in a statement that it was committed to supporting international and undocumented students.

"We are working to understand the implications on our community of the 'extreme vetting' executive order blocking immigration from certain countries,” the university said.

Related: Trump Signs Order Suspending Admission of Syrian Refugees

The order was causing confusion among travelers and at airports around the world.

A dozen people were detained at John F. Kennedy’s International Airport; two of those were Iraqi refugees, and one was later released, New York Rep. Jerrold Nadler said. A second waiver was in process for the second refugee, a senior Trump administration official said.

Protesters gathered at the airport to show support for refugees and opposition to the order. New York Gov. Andrew Cuomo, a Democrat, sharply criticized the order and said he was directing agencies to explore legal options and help anyone detained.

We are a nation of bridges, not walls. This is not who we are. And not who we should be. pic.twitter.com/ipfjkJtDOk — Andrew Cuomo (@NYGovCuomo) January 28, 2017

Trump on Saturday said of the order "it's not a Muslim ban" and that implementation of it was going smoothly.

"We're going to have extreme vetting, which we should've had in this country for many years," Trump said. A senior Trump administration official said "this is not a travel ban."