A US police federation chief called for tougher checks on gun buyers Thursday, almost a week after a shooter used four weapons, including an assault rifle, to kill 12 people at a cinema in Colorado.

Calls to re-examine America's gun laws have mounted in the aftermath of the massacre in Aurora, near Denver, during a screening of the latest Batman movie, "The Dark Knight Rises," that saw the gunman open up on the audience.

Hubert Williams, president of the Police Foundation and chairman of the National Law Enforcement Partnership to Prevent Gun Violence, said he was not calling for new laws but called for tightening up existing measures.

"America is reeling after this tragedy, with people rightly wondering why and how this could happen? And what will be done to prevent the next one, and the next one, and the one after that," he told reporters in Washington.

"Law enforcement is united on this position," Williams said. "We're asking for existing laws to be enforced for people purchasing weapons."

It has emerged that the alleged Aurora gunman James Holmes, a 24-year-old graduate student, managed to obtain part of his arsenal legally over the Internet, but controls on online arms sales are said to be lax.

Over an eight-week period, Holmes stocked up on 6,300 rounds of ammunition: 3,000 for his .233 semi-automatic AR-15 rifle, another 3,000 for his two Glock pistols and 300 cartridges for a pump-action shotgun.

The sheer firepower involved in the Aurora shooting has evoked memories of the horrific Columbine High School shootings in 1999, when two students used weapons, including automatic pistols to kill 12 contemporaries and a teacher.

The Police Federation is calling for a ban on sales of large-capacity ammunition magazines.

Holmes bought a special magazine for his AR-15 military-style assault rifle that allows a shooter to unload up to 50 to 60 rounds per minute.

"After looking at what happened in Aurora, Colorado, who could be in favor of these high capacity magazines?" Williams asked.

Such magazines "are made for battle and have absolutely no place in civilian hands on our nation's streets," he added.

According to Baltimore police chief James Johnson, the existing laws only cover the approximately 60 percent of guns that are sold by licensed dealers. The rest, he said, are sold without checks, over the Internet or at gun shows.

"How can we possibly allow nearly half the firearm transactions in this country to occur on a no-questions-asked basis?" Johnson asked.

"This is like letting 40 percent of airline passengers bypass airport security. But it's actually worse."

Since the shooting last week, New York City Mayor Michael Bloomberg has been one of the most outspoken voices in favor of stricter gun control.

But the mayor faced a backlash, including from his own party, after suggesting in an interview with CNN that police go on strike until the public forces lawmakers to "do what's required to keep us safe."

Critics have noted that police officers are legally forbidden from striking in New York, and also accuse Bloomberg of politicizing the tragedy.

In a speech on Wednesday, US President Barack Obama vowed to pursue "common sense" measures to ensure mentally unbalanced people do not get their hands on guns.

The US gun lobby, led by the National Rifle Association, is a powerful and well-funded player in Washington. It argues that clamping down on fundamental American liberties will achieve nothing.

The Law Enforcement Partnership to Prevent Gun Violence is a coalition of 10 national law enforcement groups that represent a wide range of officers and officials.