A St Louis police sergeant has been hospitalised after being shot in what the police chief described as an "ambush".

Police Chief Sam Dotson said the 46-year-old officer was shot twice in the face. The suspect got away and a massive search is underway.

Hours earlier, another police officer was shot dead in his squad car as he was writing out a traffic ticket in San Antonio, Texas.

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The officer in St Louis, Missouri, was not involved in a call or traffic stop but was sitting in traffic when another car pulled up alongside his squad car around 7.30pm on Sunday.

He told police he heard at least two shots.

"This officer was driving down the road and was ambushed by an individual who pointed a gun at him from inside of his car and shot out the police officer's window," Police Chief Sam Dotson said.

He declined to name the officer but said he is a married father of three who has been with the department for about 20 years.

He is expected to survive.

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Mayor Francis Slay said the officer did nothing to provoke an attack.

"He didn't deserve this," he said. "It looks like he's going to survive. He's going to be OK.

He added: "But this is traumatic. It's traumatic for him, his family. It's traumatic for the city of St. Louis. He was just doing his job."

Helicopters, SWAT teams and scores of officers searched the south St Louis neighborhood where the shooting occurred for a silver car.

Police were also looking for any surveillance video that may have captured the shooting.

San Antonio police investigate the scene where Detective Benjamin Marconi was shot (Edward A. Ornelas/The San Antonio Express-News via AP)

Meanwhile, the search continues for the suspect who shot and killed San Antonio police officer Detective Benjamin Michael, 50.

The 20-year veteran of the force had pulled over a vehicle and while he was inside his squad car writing a ticket, a car pulled up behind him.

The driver of the car got out, walked up to the officer's driver-side window and shot Mr Marconi twice in the head, then walked back to his car and drove away.

Hours earlier, another police officer was shot dead in his squad car as he was writing out a traffic ticket in San Antonio, Texas.

“We consider this suspect to be extremely dangerous and a clear threat to law enforcement officers and the public,” said San Antonio police Chief William McManus.

He said after the shooting officers had been instructed not to make traffic stops alone.

In July, a gunman in Dallas killed five officers who were working a protest about the fatal police shootings of black men in Minnesota and Louisiana. Ten days later, a man killed three officers in Baton Rouge, Louisiana.

Earlier this month, two Des Moines, Iowa-area officers were fatally shot in separate ambush-style attacks while sitting in their patrol cars.