CHICAGO (Reuters) - A group of civil rights activists on Tuesday filed a lawsuit against three Chicago suburbs to force them to take stricter gun-control measures to stem the flow of weapons into the city's crime-ridden neighborhoods.

The complaint filed in Cook County Circuit Court against the towns of Riverdale, Lyons and Lincolnwood said gunshops there, along with stores in Gary, Indiana, supply a fifth of guns seized by police at crime scenes in the city.

With between 400-500 murders a year, almost all with guns, Chicago lags other big U.S. cities in bringing down homicides. Police Superintendent Garry McCarthy, on the job for four years, says the biggest factor in the city's high crime rate is illegal weapons, many purchased in the suburbs.

The lawsuit brought by prominent activist Reverend Michael Pfleger of St. Sabina Catholic Church on Chicago's South Side, Reverend Robin Hood, a pastor on the West Side, and other plaintiffs alleged that the suburbs are too lax in regulating gun stores.

In Illinois, it is up to municipal governments to license and regulate gun dealers.

The lawsuit asked a judge to order the towns to force the gunshops to do background checks on their employees and to take steps to reduce gun purchases by straw men.

The lawsuit said training could help employees and managers identify common signs of someone purchasing the gun for someone else. The stores should keep a log of purchases of all guns that are later recovered in a crime, and then block sales to any customer that has purchased guns used in a crime, the complaint said.

The lawsuit alleges the three towns, located north, west and south of Chicago, have violated civil rights laws because guns purchased in stores there are used overwhelmingly in crimes that affect black communities in Chicago, bringing down property values, harming businesses and causing distress.

The Chicago Sun-Times reported that similar lawsuits filed in the past have had limited impact. The Illinois Supreme Court in 2004 dismissed a lawsuit that the City of Chicago had brought against gun dealers and others after undercover officers posed as gang members and bought dozens of guns, the Sun-Times said.

Lyons and Lincolnville village offices were closed on Tuesday evening and it was not possible to get a response to the lawsuit. "They are not giving any statements," a Riverdale employee said.

(Editing by Sandra Maler)