By Kevin Liptak, Jeff Zeleny and John Harwood, CNN

(CNN) -- President Joe Biden is expected to announce Tuesday that Merck & Co. will partner with Johnson & Johnson to help manufacture J&J's coronavirus vaccine, administration officials familiar with the matter said, increasing production capacity as the third authorized shot begins going into arms.

The arrangement between two competitors is unusual and underscores the urgency in manufacturing and distributing enough vaccine doses to inoculate as many Americans as possible.

Biden is expected to address the partnership during his remarks at the White House on Tuesday afternoon, hailing the partnership as a way to quickly jumpstart the sluggish vaccine production. Biden has promised enough vaccine doses for 300 million Americans by the end of July, though he has also said distribution challenges mean it could be longer before everyone who wants a vaccine will be able to receive one.

The planned partnership was first reported by The Washington Post.

Some details of the arrangement, including how Merck's facilities will be used and whether they will require retrofitting to produce another company's product, are still emerging. Officials suggested Biden would use the Defense Production Act to help Merck secure the required equipment and supplies to switch its facilities over to make the Johnson & Johnson vaccine.

The US Food and Drug Administration granted emergency use authorization to the Johnson & Johnson vaccine on Saturday, and the US Centers for Disease Control and Prevention recommended it for adults age 18 and older. The vaccine's distribution is already underway and first shots are expected Tuesday.

Two Covid-19 vaccines from Pfizer/BioNTech and Moderna have already been authorized for emergency use in the United States by the FDA. Unlike those vaccines, Johnson & Johnson's vaccine only requires one shot.

Merck had been working on developing its own Covid-19 vaccine, but discontinued its effort at the end of January after early studies showed immune responses were inferior to natural infection and other Covid-19 vaccines.

Merck is expected to dedicate two of its facilities to helping Johnson & Johnson, an administration official said, in a rare partnership between two competitors.

Administration officials expressed surprise and disappointment last month when it became clear Johnson & Johnson would have fewer initial doses of its vaccine than originally planned because of production issues at its facilities.

The company said it had about 4 million doses of its vaccine ready to ship "immediately," and said it should have 20 million ready by the end of March.

"We've developed an extensive partnership here in the United States and Europe and other places around the world and we're very confident in our ability to deliver 20 million doses by the end of March and 100 million doses in the first half of the year en route to a billion doses by the end of this year," Alex Gorsky, the CEO and chairman of Johnson & Johnson, said Monday on CNN.

The announcement comes as federal health officials warn against relaxing coronavirus restrictions as gains against the pandemic appear to plateau.

During a White House Covid-19 Response Team briefing Monday, CDC Director Dr. Rochelle Walensky said she is "deeply concerned" about the potential shift in the trajectory of the pandemic.

While the number of new daily cases had been declining, the most recent seven-day average of new cases -- at about 67,200 people -- represents an increase of a little more than 2% compared to the prior seven days. The most recent seven-day average of deaths has also increased more than 2%.

"Please hear me clearly: At this level of cases with variant spreading, we stand to completely lose the hard earned ground we have gained," Walensky said Monday.

Walensky said the country can stop the surge of cases in this country by wearing a mask that fits, maintaining social distance, practicing good hand hygiene and avoiding crowds.

"Please stay strong in your conviction," Walensky said. "Continue wearing your well-fitting mask and taking the other public health prevention actions that we know work."