Jan 12 (Reuters) - The U.S. government will buy 1.25 million additional doses of Regeneron Pharmaceuticals' COVID-19 antibody cocktail for about $2.63 billion, bringing the total supply of the treatment to more than 1.5 million doses.

The cocktail, a combination of two antibodies casirivimab and imdevimab, was authorized in November for emergency use by the U.S. Food and Drug Administration.

The Department of Health and Human Services said the doses will be delivered in the first half of 2021 to treat non-hospitalized, high-risk COVID-19 patients.

"With COVID-19 cases continuing to rise, treating people with mild or moderate infections can help prevent hospitalizations," HHS Assistant Secretary for Preparedness and Response Robert Kadlec said in a statement.

The Regeneron treatment, which was given to U.S. President Donald Trump during his COVID-19 infection, is part of a class of drugs known as monoclonal antibodies, which are manufactured copies of antibodies created by the human body to fight infections.

Eli Lilly's antibody treatment has also been authorized for emergency use by the regulator.

Demand for these therapies, which are given as a one-time intravenous infusion, has been disappointing. The government program is distributing and allocating the drugs.

Healthcare systems say they have been slow to ramp up use of the antibodies due to extra levels of complexity during the pandemic - including requirements for quick diagnosis times and the need to isolate infectious patients.

To date, the HHS Office of the Assistant Secretary for Preparedness and Response (ASPR) has allocated more than 127,700 treatment courses of the therapy, the agency said on Tuesday.

Regeneron's therapy will likely be effective against new variants of the coronavirus identified so far in a handful of countries, the company said earlier this week.

(Reporting by Trisha Roy and Ankur Banerjee in Bengaluru; Editing by Shounak Dasgupta and Sherry Jacob-Phillips)