This photo shows the entrance of the Natalie Medical Building at Saint Francis Hospital in Tulsa on Wednesday. AFP

The suspect, who was armed with a rifle and a handgun during his attack on the Saint Francis hospital campus, died by suicide, police said.

"Right now we have four civilians that are dead, we have one shooter that is dead, and right now we believe that is self-inflicted," Tulsa Police Department Deputy Chief Eric Dalgleish told reporters.

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He said officers responded immediately after emergency calls came in reporting that a gunman had stormed into the second floor of the Natalie Building, which houses a clinic on the Saint Francis campus.

Police "were hearing shots in the building" when they arrived, according to Dalgleish, who said officers then searched floor by floor, room by room while trying to clear the building during what authorities described as an active shooter situation.

Tulsa police cars block the entrance of the Natalie Medical Building in Tulsa, Oklahoma, on Wednesday. AFP

Earlier, police Captain Richard Meulenberg said officers were treating the scene as "catastrophic," with "several" people shot and "multiple injuries."

It was not clear how many other people might have been injured.

Dalgleish said the entire assault -- from the moment emergency calls came in, to the time officers engaged the shooter -- lasted about four minutes.

Dalgleish also noted that the suspect had yet to be identified.

US President Joe Biden has been briefed on the shooting, the White House said in a statement, adding that the administration has offered support to Tulsa officials.

Agence France-Presse