(Recasts with confirmation by TSA, adds comments by head of flight attendants' union, CDC comments, details on travel restrictions)

By David Shepardson

WASHINGTON, Aug 17 (Reuters) - President Joe Biden's administration confirmed late on Tuesday it plans to extend requirements for travelers to wear masks on airplanes, trains and buses and at airports and train stations through Jan. 18 to address ongoing COVID-19 risks.

A Transportation Security Administration (TSA) spokesperson confirmed the extension, first reported by Reuters. "The purpose of TSA’s mask directive is to minimize the spread of COVID-19 on public transportation," the spokesperson said

Major U.S. airlines were informed of the planned extension on a call with TSA and Centers for Disease Control and Prevention (CDC) on Tuesday, four people briefed on the matter said.

The current TSA transportation mask order runs through Sept 13.

The extension reflects the impact of the highly transmissible Delta variant and is an acknowledgement that transit remains potentially risky, especially for unvaccinated people.

The move comes as U.S. airlines are grappling with whether to require employees to be vaccinated, while Canada said last week https://www.reuters.com/world/americas/canada-require-vaccines-all-federal-public-servants-2021-08-13 it plans to require all airline passengers to be vaccinated.

On Friday, Homeland Security Secretary Alejandro Mayorkas told CNN there was no discussion "at this time" about requiring vaccines for domestic airline passengers.

Association of Flight Attendants-CWA President Sara Nelson said the TSA mask mandate extension "will help tremendously to keep passengers and aviation workers safe."

The current CDC order, which has been in place since soon after Biden took office in January, requires the use of face masks on nearly all forms of public transportation.

It requires face masks to be worn by all travelers on airplanes, ships, trains, subways, buses, taxis and ride-shares and at transportation hubs such as airports, bus or ferry terminals, train and subway stations, and seaports.

The requirements have been the source of some friction, especially aboard U.S. airlines, where some travelers have refused to wear masks. The Federal Aviation Administration, which has instituted a "zero tolerance" enforcement effort on unruly passengers, said on Tuesday that since Jan. 1 it has received reports from airlines of 2,867 passengers refusing to wear masks.

TSA last month told Congress that since the start of the COVID-19 pandemic there have been over 85 physical assaults on TSA officers.

In some U.S. states, transportation hubs are among the only places where masks are still required. The CDC reversed course https://www.reuters.com/world/us/cdc-set-unveil-revised-mask-guidance-vaccinated-americans-sources-2021-07-27 on July 27 and said fully vaccinated Americans should go back to wearing masks in all indoor public places in regions where the coronavirus is spreading rapidly.

The CDC recommendation currently applies to about 94% of U.S. counties.

The CDC on Tuesday cited the Delta variant's transmissibility in a statement explaining the mask mandate. "Wearing a well-fitting mask that covers your nose and mouth is a way to prevent germs from spreading between yourself and other people," it said.

A group of Republican lawmakers in July introduced legislation to prohibit mask mandates for public transport, and other Republicans want the CDC to exempt fully vaccinated Americans from the requirements.

The CDC mask order has no expiration date. The agency in June made a minor tweak to its rules, saying it would no longer require travelers to wear masks in outdoor transit hubs and in outdoor spaces on ferries and buses.

Last month, the CDC official who signed the mask order, Marty Cetron, told Reuters the transit mask mandates have been effective - and noted that children 11 and under cannot yet be vaccinated.

"Masks are really powerful and we should make sure they're part of our arsenal," Cetron said. "The truth is that the unvaccinated portion that's out there is extremely vulnerable."

United Airlines said earlier this month it will require its 67,000 U.S. employees to get vaccinated by Oct. 25.

The Biden administration, citing the highly transmissible Delta variant and rising daily COVID-19 cases https://www.reuters.com/article/us-health-coronavirus-latest/factbox-latest-on-the-worldwide-spread-of-the-coronavirus-idUSKBN2FI25S, has refused to lift any international travel restrictions that bar most non-U.S. citizens from the United States.

Airline officials think it will be weeks or even months before the administration lifts any existing travel restrictions. (Reporting by David Shepardson in Washington Editing by Sonya Hepinstall and Matthew Lewis)