A top White House official Monday said the Biden administration is working with states to improve their coronavirus vaccine sign-ups, as many Americans have expressed frustration at the difficulties in finding appointments.

"I think scheduling an appointment is too difficult — remains too difficult — in too many places," White House COVID-19 response coordinator Jeff Zients told reporters.

As states have opened vaccine eligibility to more people, signups have been plagued by technical glitches. Websites have repeatedly crashed, phone lines have been inundated with callers, and people have been unable to find open appointments.

State vaccine sign-ups are also increasingly coming under criticism for being inaccessible for the most vulnerable, including those with disabilities, people without easy access to the internet or those who can't easily navigate complicated websites, and people who speak limited English.

"Overall, too many Americans are suffering frustration, taking up way too much time to schedule an appointment," Zients said, adding that while the Biden administration has been increasing the number of vaccinators and vaccination sites, they want to help states to make sure the technology systems can handle the increased capacity and new demand.

Zients said the administration is also looking at "lower-tech solutions that the federal government might be able to provide," such as call centers or people to help navigate the system.

Zients said that “things have gotten better” in some states and localities, and that “the pharmacy systems are often better at scheduling appointments."

Still, Zients said, "Scheduling remains for far too many people too frustrating, and we need to make it better."

Zients's remarks come after the Food and Drug Administration and the Centers for Disease Control and Prevention authorized the nation's third coronavirus vaccine over the weekend. Johnson & Johnson has begun shipping a limited supply of doses that should arrive at vaccination sites as early as Tuesday.

The 3.9 million doses shipping this week is the company's entire stockpile, and there will be no additional shipments next week, administration officials said.

To date, nearly 50 million people across the country have received at least one dose of a coronavirus vaccine.