A medical helicopter evacuates victims from a "Congress on Your Corner" event in Tucson, Arizona, where U.S. Representative Gabrielle Giffords (D-AZ) among others were shot and seriously wounded, in this still image taken from video released on January 8. 2011. [Xinhua]

Six people were confirmed dead in a mass shooting on Saturday morning in Tucson, the U.S. state of Arizona, which also left 12 others injured, including U.S. Congresswoman Gabrielle Giffords of Arizona.

Giffords, who was shot in the head, has undergone a surgery and is expected to recover, Tucson Mayor Bob Walkup said at a press conference in the afternoon.

"Giffords is still alive," the mayor stressed. Some U.S. media reported earlier that the congresswoman was already dead.

Giffords was shot one time and the bullet went through her head, according to a spokesman at the hospital where Giffords is being treated.

"The congresswoman is not deceased," but is in critical condition and "is under intensive care" after the surgery, said the spokesman.

The doctor treating Giffords for the gunshot wounds, Dr. Peter Rhee at Tucson's University Medical Center, said at the press conference that he was quite "optimistic" about her recovery.

Meanwhile, authorities confirmed that the shooting had at least 18 victims, with six of them already dead.

U.S. District Judge John Roll and a nine-year-old girl were among those killed, U.S. President Barack Obama said in a statement, calling the incident a "tragedy for our entire country. "

The president added that Giffords was still "battling for her life" and that the motive of the attack remained unknown.

Giffords' spokesman C.J. Karamargin said an unspecified number of the congresswoman's staff members were injured in the shooting.

Congressional officials said an aide to Giffords was killed, but few details were released about the Giffords staff members.

U.S. Representative Gabrielle Giffords (D-AZ), who was shot during an appearance in Tucson, Arizona is seen in an undated handout photo provided by her Congressional office in Washington, January 8, 2011. [Xinhua]

Giffords was meeting with her constituents at a public event outside a grocery store in Tucson when the shooting occurred at about 10:00 a.m..

Arizona law enforcement officials said a gunman in the crowd shot Giffords, a 40-year-old Democrat. Witnesses said they heard 15 to 20 shots.