Three gunmen have opened fire at a shopping centre in San Antonio in the US state of Texas, injuring a number of people and sparking panic.

An emergency situation is under way at South Park Mall in the city's south in the wake of the shooting, which occurred just before 9pm local time (2pm AEDT).

San Antonio Police Department said witnesses reported seeing three suspects open fire on a group of people before jumping in a black Dodge Charger vehicle and fleeing the scene.

Initial reports indicate four people have been injured, one of them critically, and are being treated at a local hospital.

According to TV reporter Matthew Seedorf, the ages of the victims are 17, 19, 26 and 41.

Harrowing vision captured by an onlooker and shared to social media shows shoppers ducking for cover near an entrance.

Julian Castro, running for the Democratic nomination for president, also a San Antonio native, expressed his shock.

"As we await more information, my heart goes out to the victims, their families, our city, and our country," Mr Castro tweeted. "Enough is enough - we must act on gun reform now."

Local journalist Jacob Beltran tweeted that witnesses heard arguing just moments before shots rang out.

Locals have been urged to stay away from the shopping centre and police have established a crime scene.

A manhunt for the gunmen is now under way.

More to come …