The US Federal Trade Commission (FTC) is expected to file a lawsuit as early as Tuesday to block Amgen Inc's $27.8 billion deal to buy Horizon Therapeutics PLC, a person familiar with the matter told Reuters.

Last year, Amgen agreed to buy Horizon to strengthen its rare diseases drugs portfolio and said it hopes to complete the deal in the first half of 2023.

Amgen said on Monday it wasn't aware of any decision made by the agency.

US Senator Elizabeth Warren, an outspoken critic of corporate consolidation, wrote to the agency earlier this year expressing her concerns on pharmaceutical deals including the Amgen deal, citing threat to competition in the sector.

The deal would give Amgen two fast-growing drugs, the thyroid eye disease treatment Tepezza and gout treatment Krystexxa. Amgen hopes they can act as a bulwark against competition for its blockbuster arthritis drug Enbrel.

(Reporting by Akanksha Khushi in Bengaluru and Diane Bartz; Editing by Sherry Jacob-Phillips)