A viral outbreak has killed six children and left 12 more sick at a rehabilitation centre in New Jersey, state authorities said on Tuesday.

The New Jersey (NJ) Department of Health confirmed 18 cases, of which six were fatal, of adenovirus among paediatric residents at the Wanaque Center for Nursing and Rehabilitation in Haskell, about 40 kilometres north-west of New York City.

Adenoviruses are a family of viruses that often cause mild illness, particularly in young children.

The strain in this outbreak, adenovirus 7, has been "particularly associated with disease in communal living arrangements and can be more severe," the department said in a statement.

Authorities characterized those affected as "medically fragile children with severely compromised immune systems."

NJ Department of Health commissioner Shereef Elnahal said on Twitter that the centre and authorities are working together to minimize the impact of the outbreak "on patients and staff."

"We take this very seriously," Elnahal said.

During an inspection on Sunday, a team from the Health Department found "minor hand washing deficiencies" at the facility, the statement said.

The centre has been instructed by authorities not to admit any new patients "until the outbreak ends and they are in full compliance."

"Heartbroken that several children have lost their lives in an adenovirus outbreak in Haskell," said Phil Murphy, the state governor, in a tweet.

"The NJ Department of Health will continue its active on-site surveillance and has recommended measures to protect against the further spread of infection."

The outbreak investigation is ongoing, authorities said.