Jan 4 (Reuters) - BlackRock Inc (BLK.N) and American Express Co (AXP.N) are extending their hybrid work plans as the Omicron COVID-19 variant spreads across the United States. The world"s largest asset manager, BlackRock, is providing flexibility through Jan. 28 and allowing U.S. employees to work from wherever they are most comfortable, according to a source familiar with the matter. The new variant has swiftly spread across the country since its detection on Dec. 1, replacing Delta as the dominant strain and sparking a new wave of infections that pushed daily cases near the 1 million mark on Monday. read more BlackRock had earlier required more than half of its employees to work from office for three days a week on average starting November. (https://reut.rs/32PBvzV) AmEx has decided to delay the Jan. 24 launch of "Amex Flex" in the United States, after previously saying it would start bringing its employees in the United States, the UK and Germany back to offices from Jan. 24. read more A number of Wall Street banks and investment firms, including Bank of America (BAC.N), Citigroup (C.N) and Jefferies Financial Group (JEF.N) have reversed their push to get staff back to the office amid concerns of the fast-spreading Omicron variant.