Image copyright Martin G. Johnson / YouTube Image caption Video uploaded to social media showed residents confronting the police about the unmarked truck

Police in the US city of Chicago say they will "take a hard look" at their use of "bait trucks" to catch thieves following criticism from activists.

Video posted on social media last week showed residents confronting police officers about a white unmarked truck.

Activists say it was filled with designer shoes and left partially open to entrap groups of young people.

But the police say the truck was used to target thieves who had been breaking into freight containers at a rail yard.

"At the end of the day, just because it's out there and it's not yours, that doesn't mean you're supposed to take it," police Superintendent Eddie Johnson said at a news conference on Thursday.

"But as [the] Chicago Police Department, we're going to take a hard look and see if there's something we can do better."

The truck was used as part of a joint surveillance operation with Norfolk Southern railways that was reportedly nicknamed Operation Trailer Trap.

A spokeswoman from Norfolk Southern said the aim of the operations was to combat "unacceptable thefts" and "apprehend the individuals who have been breaking into freight containers at [our] rail yards in Southside Chicago."

A number of arrests were made following the operation, the company said.

But community groups say the truck was parked in low income, predominantly black, areas of the city and risked worsening the relationship between residents and the police.

Image copyright Getty Images Image caption Police Superintendent Eddie Johnson and his department have been accused of engaging in stunts

"Police in Chicago must focus on building trust and better relationships within the communities they serve, not engage in stunts like bait trucks," Karen Sheley of the American Civil Liberties Union (ACLU) said in a statement.

"[The police department] admits that it can't solve murders and violent crimes because communities of colour don't trust the Chicago Police. These stunts won't help."

Others said the truck was a waste of police resources. Roderick Sawyer, who chairs the city council's black caucus, slammed the operation as an attempt to "create crime".

"This initiative serves only to undermine already fragile efforts to build trust between law enforcement and the community," he said.

He added: "These sort of tactics are the last thing we should be spending manpower and energy on."

What are "bait trucks"?

It is not uncommon for police to use vehicles and packages to lure potential thieves.

The most common example is police "bait" cars, which are typically parked in areas where vehicle crime is a problem.

The specially-equipped cars are sometimes left with mobile phones, sat navs, laptops and other valuables on display.

They are usually equipped with GPS tracking devices so the police can find the stolen goods and apprehend the thief.

Earlier this year, police in San Francisco deployed "bait bikes" following a surge in thefts in the city.

In 2012, police in the UK filled a lorry with fake goods and used it as bait to target gangs.