The Biden administration is facing intense scrutiny from a small group of Senate Democrats about the administration's Covid-19 pandemic response, mainly the testing shortage as cases of the Omicron variant have surged across the United States.

In a letter to White House Covid-19 response coordinator Jeffrey Zients on Thursday, led by Nevada Sen. Jacky Rosen, the groups asks, "why the Administration failed to take more significant steps earlier to increase access to at-home tests."

While praising the administration's efforts to promote and distribute vaccines, the group is fiercely critical of the slowness to get tests out across the country.

"Far too many measures -- such as increasing access to home-based testing -- have been reactive, rather than proactive," the letter reads.

"This Administration either knew or should have known that testing shortages were occurring across the country over the past several months, and with the full expectation that the virus would likely mutate into a new variant steps to increase testing access should have happened before the current wave hit, not several weeks into the surge, with resources still not available until later this month or beyond."

The senators are calling on the administration to improve preparedness to ensure they are not caught off guard again and asking if they need Congress' help to address the current spike in cases.

The letter is also signed by Sens. Mark Kelly of Arizona, Joe Manchin of West Virginia, Kyrsten Sinema of Arizona and Jon Ossoff of Georgia.

A White House official told CNN in a statement Friday evening that "we are in contact with our colleagues at the Hill about a range of issues related to our COVID-19 response, and share their goal of expanding access to free, at home testing" and cited administration efforts such as increasing the number of tests on the market since August and using the Defense Production Act "regularly to accelerate manufacturing."

"While we are not where we would like to be with at home testing, neither are most countries due to unprecedented demand," the official said. "Every day more supply arrives on the market. We are looking forward, and are encouraged by our actions, which are now allowing the President to purchase an unprecedented 1 billion at home tests that will be made available to the American people for free."

On Monday, a group of 50 lawmakers, including some high-profile Biden allies, such as independent Sen. Bernie Sanders of Vermont and Democratic Rep. Adam Schiff of California, also wrote a letter to President Joe Biden, urging "additional, immediate steps" to ramp up testing supply so that every American has access to one rapid test a week.

Since the holidays, Covid-19 at-home tests have been difficult to find and there have been long lines at many testing facilities across the US.

Biden announced Thursday that he's directing his team to procure another 500 million Covid-19 rapid tests in addition to the 500 million tests he announced last month would be distributed for free nationwide.

Biden also said the website where Americans can sign up for free tests to be mailed to them will be rolled out next week. More details are expected Friday, according to a White House official.

But the administration is still in the process of acquiring the first half-billion Covid-19 tests and only last week began signing contracts to procure them. The White House said it expected the first free rapid tests to be delivered to the government "early next week."

This story has been updated with additional details Friday.