A US appeals court has questioned whether President Donald Trump's executive order specifically targets Muslims, and will now decide whether a temporary block should remain in place.
At a hearing at the ninth Circuit Court of Appeals in San Francisco, government lawyers argued that the president should have the authority to decide who can enter the country.
Lawyers for the states of Washington and Minnesota, who last week successfully managed to halt the executive order, were questioned on whether the travel ban could be discriminatory if it affected only 15 per cent of the world's Muslims.
A ruling is expected soon, and the case could end up in the Supreme Court.
Harry Horton reports from Washington
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A courtroom drama on Donald Trump's most ambitious and controversial piece of legislation so far.
In an hour long oral hearing - unusually conducted by telephone - lawyers representing the government argued that the executive order was in the interests of national security.
But judges on the court questioned what evidence there was that the seven countries targeted by Donald Trump's travel ban posed a threat to the United States.
Here's US Justice Department Lawyer August Flentje
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"These proceedings have been moving very fast. And the strongest point on that is that in 2015 and 2016, both Congress and the administration made determinations that these seven countries posed the greatest risk of terrorism and in doing so restricted visa waiver to people who would even traveled to those countries over the last five or six years."
The judges asked whether states have the power to halt a president's executive orders.
The attorney representing the states of Washington and Minnesota - Noah Purcell - argued that the travel ban specifically targeted Muslims - and was therefore unconstitutional.
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"I mean there are statements that we've quoted in our complaint that are rather shocking evidence of intent to discriminate against Muslims, given that we haven't even had any discovery yet to find out what else might have been said in private. I mean, the public statements from the president and his top advisers reflecting that intent are strong evidence, certainly at this pleading stage to allow us to go forward on that claim."
Both lawyers faced tough questions and the judges promised to deliver their verdict quickly.
Meanwhile, with the travel ban temporarily lifted, immigrants from the affected countries continue to travel to the United States.
Whichever side loses this case, an appeal looks likely - and Donald Trump's immigration order will probably end up in the Supreme Court.
Harry Horton, Washington