WASHINGTON - A US hospital was put on emergency lockdown on Saturday after 'several patients' arrived at a St. Louis, Missouri emergency room with apparent exposure to an unknown substance, the hospital said.

A local television station reported that two people were in critical condition on Saturday after at least two hospitals in or near St. Louis received patients complaining of exposure, and that hazardous materials teams had begun decontamination of several dozen people quarantined after having come in contact with the patients.

'Several patients came to St. Anthony's Emergency Department from a site in East St. Louis, Illinois with an unknown chemical exposure,' St. Anthony's Medical Center spokesman Mary Jo Wich said in a statement on the hospital's website.

'As a precaution for our patients, visitors and employees, access to the entire hospital campus initially was restricted,' she said. As of 2315 GMT restrictions on all but the emergency department had been lifted.

The emergency department 'will remain closed until the nature and extent of the exposure is determined,' Ms Wich said.

Local TV station KMOV, citing officials, reported that a second hospital, SSM DePaul Health Center in Bridgeton, Missouri near St. Louis, was also placed on lockdown after three people entered that facility claiming exposure to an unknown white powder, and that two of the patients were in critical condition.

KMOV reported that the cases were likely linked to people who were exposed to a substance while rooting through a dumpster in East St. Louis.

But officials were not saying what that substance may have been, according to KMOV, which aired images of dozens of firefighting crews, paramedics and emergency vehicles at the DePaul Health Center.

News of the exposures emerged as Republican presidential hopeful John McCain was preparing to fly to St. Louis ahead of a rally on Sunday in nearby O'Fallon, Missouri.

There is 'no change of plans at this point' Mr McCain spokesman Kimmie Lipscomb told AFP. -- AFP