President Donald Trump has signed a proclamation to suspend international visas for new students at Harvard University, the White House announced Wednesday, in a dramatic escalation of its efforts to block foreign nationals from enrolling at the elite university.

The Trump administration has tried repeatedly to restrict Harvard's ability to enroll international students — a serious threat for a school where more than one in four students hail from foreign countries .

Harvard University said on Thursday that U.S. President Donald Trump's move to bar foreign nationals seeking to study at the Ivy League school from entering the United States is illegal, and asked a judge to block it immediately pending further litigation.

The university called it “yet another illegal retaliatory step” from an administration that has sought ways to circumnavigate the courts in its push to bar foreign students.

The move to limit student visas at Harvard comes days after a judge blocked the Trump administration’s attempts to revoke the school’s ability to host foreign students.

The efforts by President Donald Trump’s administration have struck at Harvard's identity and unsettled current and prospect students around the world.

The Trump administration continues to target foreign nationals looking to attend U.S. universities. Here's a closer look at who these students are, where they come from and what they study.