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.

A group of 18 leading US research universities, including Princeton, MIT, Caltech and Johns Hopkins, asked a federal judge for permission to file legal arguments in support of Harvard University in its high-stakes showdown with the Trump administration over more than $2 billion in frozen grant money.