By David Shepardson and Tracy Rucinski

WASHINGTON/CHICAGO (Reuters) - The U.S. Treasury said on Tuesday it had closed loans to seven large airlines hit hard by the coronavirus pandemic and urged Congress to save tens of thousands of airline jobs by extending billions in payroll assistance.

The Treasury said in a statement the seven carriers were Alaska Airlines, American Airlines, Frontier Airlines, JetBlue Airways Corp , Hawaiian Airlines, SkyWest Airlines and United Airlines.

Airlines and unions were still heavily lobbying Congress ahead of a Wednesday deadline for a new $25 billion bailout to keep workers on the payroll for another six months, but industry officials acknowledge they face an uphill battle with just hours left.

U.S. airlines received $25 billion in March under the CARES Act, primarily in the form of grants to keep employees on payroll through September and avoid furloughs.

Treasury Secretary Steven Mnuchin urged Congress on Tuesday to extend the payroll assistance program "so we can continue to support aviation industry workers as our economy reopens and we continue on the path to recovery." Last week, Mnuchin ruled out executive action to avert airline layoffs.

House of Representatives Democrats have backed a $2.2 trillion measure that would provide assistance to many hard-hit sectors as well as direct relief for Americans. They have been reluctant to support a stand-alone measure that would only aid airline workers.

Nick Calio, who heads the airline trade group Airlines for America, said Tuesday that carriers "remain hopeful that Congress will act quickly to save these jobs before the Sept. 30 deadline – which is tomorrow – but time is running out."

American Airlines said Friday it secured a $5.5 billion Treasury loan and could tap up to $2 billion more in October.

Airlines, which together continue to burn through $5 billion a month, have until Sept. 30 to decide whether to draw from the $25 billion U.S. Treasury loan fund, which was authorized by Congress in March.

Treasury said it expects "the initial loan amounts will be increased as a result of some major airlines determining not to move forward with loans" and could boost loans to as much as $7.5 billion per carrier.

Airlines have also tapped capital markets to bolster liquidity, but passenger traffic still down 68% from pre-pandemic levels.

(Reporting by David Shepardson and Mohammad Zargham; Editing by Eric Beech and Leslie Adler)