Washington DC - A panel of skeptical US appeals court judges grilled a government lawyer Thursday on President Donald Trump's use of an emergency authority to impose sweeping tariffs against countries around the world.

The court hearing came as the clock ticked down to a midnight deadline for dozens of economies – including key commercial partners like Canada – to secure US tariff deals.

The federal Court of International Trade blocked most of the duties from taking effect in May, but the US Court of Appeals for the Federal Circuit put the ruling on hold to consider the case.

Assistant Attorney General Brett Shumate, representing the Trump administration, defended the president's authority to impose the tariffs under the 1977 International Emergency Economic Powers Act (IEEPA).

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Shumate said they were needed to address the "exploding trade deficit."

"Congress has long given the president broad discretion to deal with complex and evolving national emergencies in light of his preeminent role in foreign affairs and national security," the Justice Department official said.

"For decades, IEEPA has been one of the most powerful tools that any president can use to protect our national security, on foreign policy and our economy," Shumate said.

Neal Katyal, a former Obama solicitor general representing the small businesses who brought the suit, said Trump's tariff moves constituted "a breathtaking claim to power that no president has asserted in 200 years."

"The president can do whatever he wants, whenever he wants, for as long as he wants, so long as he declares an emergency," Katyal said.

"If the government is going to have that power, then Congress has to give it to them clearly."

A majority of the judges on the appeals court panel appeared skeptical of the government's arguments, and the case is likely to end up in the Supreme Court, where conservatives enjoy a 6-3 majority.

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Trump took to Truth Social ahead of the hearing, wishing his lawyers "good luck in America's big case today."

"If our Country was not able to protect itself by using tariffs against tariffs, we would be 'dead,' with no chance of survival or success," he said.