WASHINGTON: US President Barack Obama has ordered far-reaching changes to the US immigration system that will protect nearly five million people from deportation, testing the limits of his presidential powers and inviting a showdown with newly emboldened Republicans.

Obama sought to break a stalemate in America's long-simmering debate over immigration by cutting out Congress, confronting Republicans who swept congressional elections earlier this month and ensuring that the contentious debate will carry on into the 2016 presidential campaign.

In a televised address, on Thursday night, Obama described the most sweeping changes to fractured immigration laws in nearly three decades, saying his executive actions were a “common sense” plan consistent with what previous presidents of both parties had done.

Immigrants living illegally in the United States would be saved from deportation by receiving work permits; millions more would remain in limbo.

“To those members of Congress who question my authority to make our immigration system work better, or question the wisdom of me acting where Congress has failed, I have one answer: Pass a bill,” Obama said.

Republicans, who take full control of Congress in January after capturing the Senate from Democrats, warned that Obama would face serious consequences for what they described as an unconstitutional power grab.

“The president will come to regret the chapter history writes if he does move forward,” declared Sen. Mitch McConnell, the Republican who is soon to become the Senate majority leader, hours before Obama's speech.

Republicans were united in opposing his move but divided on how to respond.

Details of Obama's immigration plan

Some five million people, 44 per cent of the estimated 11.3 million immigrants living illegally in the US will benefit from this move.

The nation's undocumented people hail mostly from Mexico and Central America, and 60 per cent of them live in just six US states: California, Florida, Illinois, New Jersey, New York and Texas.

Half of them have lived in the US for 13 years or more.

Authorization for three years

A new deferred action program will shield up to four million undocumented immigrants from deportation, allowing them to apply for three-year work permits.

Those eligible must have been in the United States for more than five years, and be a parent of a US citizen or of a legal permanent resident born on or before November 20, the date of Obama's order.

Applicants must have a clean criminal record, pass a background check and pay taxes. They will also need to cover a processing fee of $465.

The Department of Homeland Security will begin accepting applications next spring.

Expanded path for young migrants

Obama is broadening the Deferred Action for Childhood Arrivals (DACA) program he created in June 2012 specifically for children, potentially adding 270,000 youths to the 600,000 already legalized by that program.

Children brought to the country before January 1, 2010 are now eligible, compared with DACA's original June 15, 2007 cutoff.

The maximum age of 31 in the original program is removed, with no new age limit imposed.

As before, applicants must have arrived in the US before age 16.

Applicants must be in school or have a high school degree or equivalent, or have been honorably discharged from the military, and have a clean criminal record.

Work authorisation is expanded to three years instead of two.

Deportation

Federal authorities will shift deportation efforts from a broad dragnet to “much more sharply defined” priorities, a senior administration official said.

Those deportation targets include undocumented immigrants convicted of serious crimes, and people who recently crossed the border illegally.

Additional resources will help officials reinforce border security, speed deportation of border crossers, and streamline the immigration court process to help clear a backlog of pending cases.

Legal immigration

The administration is poised to facilitate visas for highly-skilled workers and science and technology students, a move that could impact approximately 500,000 people.

Foreign graduates of US universities in STEM fields- science, technology, engineering and mathematics will be allowed to work longer in country after earning degrees, without needing a new visa, through the “Optional Practical Training” extension program.