Malala Yousafzai, the Nobel Peace laureate, has said she was "heartbroken" by Donald Trump's order on refugees and urged the US president not to abandon the world's "most defenseless."

"I am heartbroken that today President Trump is closing the door on children, mothers and fathers fleeing violence and war," said Malala.

"In this time of uncertainty and unrest around the world, I ask President Trump not to turn his back on the world's most defenseless children and families," she added in a statement just moments after Trump signed the decree.

Malal Yousafzai is the youngest-ever recipient of the Nobel Peace Prize, which she shared in 2014 with India's Kailash Satyarthi, a fellow education activist.

"I am heartbroken that America is turning its back on a proud history of welcoming refugees and immigrants - the people who helped build your country, ready to work hard in exchange for a fair chance at a new life," she said.

The decree signed by Trump was entitled "Protection of the nation from foreign terrorist entry into the United States." The White House did not immediately make the wording public, but a draft text leaked to US media said it would suspend the entire US refugee resettlement program for at least 120 days while tough new vetting rules are established.