Health officials have identified clusters of an unusual virus which appears to strike children but not adults, across several US states.

Clusters of severe respiratory illness across a dozen US states has raised concerns about the spread of an unusual virus that is striking children but not adults.

The culprit in most cases identified so far is enterovirus 68, or EV-D68, said assistant surgeon general Anne Schuchat.

She said "about 12 states" have reported apparent clusters to the US Centers for Disease Control and Prevention.

"The situation is evolving quickly," she said on Monday. "I wouldn't be surprised if there are many more."

No deaths have been reported this year, but infants, children and teenagers are most vulnerable and some states have seen a spike in hospitalisations, she said.

"This isn't a new virus but EV-D68 is fairly uncommon and we don't know as much about it as we do about some of the other common enteroviruses," Schuchat told reporters.

"It can cause mild or severe respiratory illness, yet the full spectrum of all of the illnesses it can cause are not well-defined and that is something people are looking into with these clusters."

Clusters of EV-D68 have also been seen recently in Japan and the Philippines, she told reporters.

While EV-D68 was reportedly uncovered in some tests in children who were afflicted with a polio-like syndrome earlier this year, Schuchat said no neurological symptoms have been seen in this summer's US cases.

The virus likely spreads from person to person when an infected person coughs or sneezes, or when someone touches an infected surface.

There is no vaccine against enterovirus-D68, and experts do not understand why it is emerging and spreading now.

Copyright 2014 AAP