While there has been reports of spike in hospitalisation due to Omicron in South Africa, according to President Cyril Ramaphosa, they are "not alarming".
"Though it's too early to really make any definitive statements about it, thus far it does not look like there's a great degree of severity to it," Fauci was quoted as saying on CNN's 'State of the Union' on Sunday.
"Thus far, the signals are a bit encouraging. But we have really got to be careful before we make any determinations that it is less severe, or it really doesn't cause any severe illness, comparable to Delta," he added.
Lab tests are underway to determine whether the super mutant Omicron is more transmissible than other strains, resistant to immunity from vaccination, and if infection is more severe. The results are expected within weeks.
Meanwhile, at least 15 states in the US have detected the Omicron variant and that number is expected to rise, according to Centers for Disease Control and Prevention Director, Rochelle Walensky.
"We know we have several dozen cases and we're following them closely. And we are everyday hearing about more and more probable cases so that number is likely to rise," Walensky was quoted as saying on ABC News' 'This Week'.
Even if Omicron proves less dangerous than Delta, it remains problematic, World Health Organization epidemiologist Maria Van Kerkhove told CBS' 'Face The Nation'.
"Even if we have a large number of cases that are mild, some of those individuals will need hospitalisations," she said.
"They will need to go into the ICU and some people will die. We don't want to see that happen on top of an already difficult situation with Delta circulating globally."
The US, last week, along with more than 50 countries imposed a travel ban on South Africa and seven other southern African countries to stem the variant's spread. However, scientists say that the travel restrictions have come too late and could even slow studies of the new super mutant.
Fauci said the US will likely lift its ban on travellers from southern African countries in a "reasonable period of time".
However, the vast majority of cases in the US continue to be caused by the Delta variant.
"We have about 90 to 100,000 cases a day right now in the US, and 99.9 per cent of them are the Delta variant," Walensky said.