US President Donald Trump has signed an executive order that indefinitely bans Syrian refugees from entering the US and temporarily blocks entry to people from six other Muslim-majority countries.

The Republican's "extreme vetting" order is intended to fight radical Islamic terrorism, he said, as he put a 90-day block on US entry to people from Iran, Iraq, Syria, Yemen, Sudan, Libya and Somalia, and a currently unlimited ban on refugees fleeing Syria.

"I'm establishing new vetting measures to keep radical Islamic terrorists out of the United States of America," Trump said when signing the executive order, The Hill reported.

"We don't want them here. We want to ensure that we are not admitting into our country the very threats our soldiers are fighting overseas. We only want to admit those into our country who support our country and love deeply our people."

The executive order, titled "Protecting the nation from foreign terrorist entry into the United States" says: "Numerous foreign-born individuals have been convicted or implicated in terrorism-related crimes since September 11, 2001, including foreign nationals who entered the United States after receiving visitor, student, or employment visas, or who entered through the United States refugee resettlement program.

"Deteriorating conditions in certain countries due to war, strife, disaster, and civil unrest increase the likelihood that terrorists will use any means possible to enter the United States."

It outlined proposals including the temporary ban on individuals from six countries, the indefinite ban on Syrina refugees from entering the US, and reiterated comments made by Trump earlier in the day on the Christian Broadcasting Network that suggested priority would be given to those fleeing religious persecution.

The order stated the US would: "prioritise refugee claims made by individuals on the basis of religious-based persecution, provided that the religion of the individual is a minority religion in the individual's country of nationality".

The nature of the executive order has already faced criticism, with some viewing it as a Muslim ban, the idea for which was floated by Trump during his presidential campaign.

American Civil Liberties Union Executive Director Anthony Romero said in a statement seen by Fox News: "'Extreme vetting' is just a euphemism for discriminating against Muslims.

"Identifying specific countries with Muslim majorities and carving out exceptions for minority religions flies in the face of the constitutional principle that bans the government from either favoring or discriminating against particular religions," he added.