The Trump administration is continuing its talks with trading partners despite a U.S. appeals court ruling that most of President Donald Trump's tariffs are illegal, U.S. Trade Representative Jamieson Greer said on Sunday.

"Our trading partners, they continue to work very closely with us on negotiations," he said in an interview on Fox News' "Fox & Friends" program.

"People are moving forward with their deals, regardless of what this court may say in the interim."

Greer did not say which countries the United States was still in talks with, but said he had spoken with one trade minister on Saturday morning.

The ruling threatens what has become a pillar of Trump's foreign policy since starting his second term in the White House in January. He has used the levies imposed on imported goods to exert political pressure and renegotiate trade deals even as the tariffs have increased volatility in financial markets.

The U.S. Court of Appeals for the Federal Circuit in Washington's 7-4 decision on Friday said while Congress gave the president significant authority to act in response to a national emergency, lawmakers did not "explicitly include the power to impose tariffs, duties, or the like, or the power to tax."

The decision addressed the legality of Trump's so-called reciprocal tariffs set in April as well as tariffs imposed against China, Canada and Mexico in February, but does not impact those issued under other legal authority.

Trump on Friday blasted the decision and vowed to take the case to the U.S. Supreme Court. The appeals court said his tariffs can remain in effect through October 14 to allow for appeals.

Trade experts said the Trump administration had been bracing for the ruling and preparing alternative plans to be able to proceed with its tariffs.

"If other countries are looking at this and thinking they're going to get tariff relief, they're in for a unpleasant surprise. There are backup options upon backup options, even if the Supreme Court ends up agreeing with the appeals court," said Josh Lipsky, chair of international economics at the Atlantic Council think tank.

He said one option would be to turn to Section 338 of a 1930 trade law that allows the president to impose duties of up to 50% against imports from countries that are found to discriminate against U.S. commerce.

Trump weighed in again on Saturday in a social media post, saying "A big year ahead for the USA, maybe the BEST EVER, if the Tariffs are finally approved by the Courts!!!"

The president headed to his Virginia golf club on Sunday, ahead of Monday's Labor Day holiday celebrating U.S. workers.

White House trade adviser Peter Navarro told Fox News' "Sunday Morning Futures With Maria Bartiromo" that the administration was optimistic the 6-3 conservative Supreme Court would back Trump's tariffs.

Fellow Republican U.S. Senator James Lankford said companies he has talked to want the issue settled.

"Every time there's a new court hearing, every time there's a new change, it's destabilizing for every one of our businesses. So let's get all these things resolved as quickly as we possibly can," he told NBC News' "Meet the Press."