CHICAGO (AFP) - An Uber driver was charged with six counts of murder after he allegedly went on a weekend killing spree in the US state of Michigan - possibly picking up passengers along the way.

Jason Brian Dalton, 45, sat stony-faced in an orange prison jumpsuit, thick glasses shielding his downcast eyes, as a judge read the charges against him in a Kalamazoo court on Monday (Feb 22).

"I wish to remain silent," Dalton said via videoconference from the jail when asked if he had anything to tell the court.

Prosecutors said they were still trying to determine why Dalton began firing - seemingly at random - as he drove through Kalamazoo on Saturday night.

"That is probably the million dollar question, 'Why would this individual do this?'" public security chief Jeffrey Hadley told CNN.

The first victim was a young woman who was with her three children outside an apartment complex when she was shot at 6pm local time. She was seriously wounded but is expected to survive.

Four hours later, the gunman opened fire at a car dealership, killing a father and his 18-year-old son.

The last and deadliest shooting came 10 to 15 minutes later at a Cracker Barrel restaurant, where four more people were killed and a 14-year-old girl was critically wounded, officials said.

Dalton was also charged with two counts of assault with intent to murder and eight weapons offences.

Mr Hadley described Dalton, a former insurance adjuster, as "an average Joe" who had no criminal record and had not come to the attention of law enforcement before the murders.

The New York Times said neighbours described him as quiet and polite, but that he occasionally fired a gun outside the back door of the house he shared with his wife and two children, aged 10 and 15.

His family released a statement offering their "deepest sympathies and condolences to the families of the victims" and vowed to "cooperate in every way that we can to help determine why and how this occurred".

"There are no words which can express our shock and disbelief," read the statement.

"While it seems woefully inadequate, we are deeply sorry and are praying for everyone affected."

The police are looking into reports that Dalton picked up fares between shootings.

A man who declined to be identified told CNN that he got a short ride from Dalton in the midst of the rampage.

"I said 'You're not the shooter, are you?' He said no. I said, are you sure? He kind of just said, 'No, I'm just tired, I've been driving for seven hours.'"

Uber's tracking system is expected to help police trace Dalton's vehicle during the series of shootings.

The controversial ride-sharing company unveiled a new global code of conduct and created worldwide "incident response teams" last year to reassure riders who have become increasingly worried for their safety after several high-profile incidents.

Uber said Dalton had passed a background check and had no criminal record prior to joining the company.

One passenger has described being taken on a wild ride by Dalton a few of hours before the rampage began.

"We were driving through medians, driving through the lawn, speeding along and when we came to a stop, I jumped out the car and ran away," Mr Matt Mellen told CBS affiliate WWMT.

He said he escaped at 4.30pm and called the emergency phone number 911. He told his fiancee what had happened and she posted Dalton's picture on Facebook as a warning to others.

The Kalamazoo killings were the latest in a string of US mass shootings that include the Dec 2 massacre in San Bernardino, California that left 14 people dead, and the Dec 14, 2012, Sandy Hook school massacre that killed 26, including 20 children.

Gun violence claims the lives of about 30,000 Americans every year and mass shootings - rare in most countries - have been on the rise.

In what has become a grim tradition, President Barack Obama on Monday decried the nation's epidemic of gun-related deaths and said more needs to be done to keep guns away from "dangerous people".

"Their local officials and first responders did an outstanding job in apprehending the individual very quickly, but you've got families who are shattered today," he told a White House gathering of US governors.

"Clearly we're going to need to do more if we're going to keep innocent Americans safe."

Michigan Governor Rick Snyder ordered flags lowered for six days in honour of each of those killed and met relatives of hospitalised victims.

"What a tough situation," he told reporters as he described watching the wounded girl's mother react as her critically injured daughter squeezed her hand. "Such senseless violence."

Dalton was ordered held without bail ahead of a preliminary hearing set for March 3.