Two US nuclear power plants were shut down early Tuesday in the aftermath of superstorm Sandy, but the plant operators stressed there were no risks to the public.

New Jersey's main power company PSEG Nuclear shut down its Salem 1 unit on the Delaware river, saying most of its water circulation pumps had been rendered unusable "due to weather impacts".

PSEG said it manually shut down the 1,175 MW unit, but said there were "no issues" in the shutdown and the facility was "currently stable."

The Salem 2 unit was already offline for maintenance when the storm hit, and PSEG said another nearby nuclear unit, Hope Creek, remains operating at full power.

In New York, power generator Entergy shut down a unit of its Indian Point nuclear facility "due to external electrical grid issues."

Another unit remained at full power, and Entergy said on Twitter that there was "no risk" to the public or company employees.

Late Monday, the Nuclear Regulatory Commission said it was monitoring all plants in the US northeast as Sandy pushed up water levels in rivers and bays, which are crucial for cooling operations at the facilities.

The Oyster Creek plant in New Jersey, which was in a regularly scheduled outage when the storm hit, set off an alert when floodwaters exceeded a threshold set for its water intake facilities.