A gunman has shot and killed eight people shopping at a mall north of Dallas, Texas, emergency services say.

Hundreds of people were evacuated from the mall in the city of Allen, as eyewitnesses described a man firing indiscriminately at passers-by.

Police said they shot the gunman dead and believe he acted alone.

Some of the victims are thought to be children. At least seven people are being treated in hospital, of whom three are critically ill.

Allen Fire Chief Jonathan Boyd said seven people - including the gunman - were pronounced dead at the scene and two died later in hospital.

A police officer "heard gunshots, went to the gunshots, engaged the suspect and neutralised the suspect," Allen Police Chief Brian Harvey said.

Texas Governor Greg Abbott described the shooting as an "unspeakable tragedy" and said the state was ready to offer any assistance to local authorities.

Police have asked members of the public with video captured at the scene to contact the FBI, as they collect evidence.

One eyewitness, Fontayne Payton, told the AP news agency he heard gunshots though his headphones as he shopped at H&M.

When people were allowed to leave the mall, he described seeing bodies outside.

"I pray it wasn't kids, but it looked like kids," he said. "It broke me when I walked out to see that."

One man told CBS News he heard "pops go off... and there was a guy right across, near like Francesca's, in a full outfitted assault rifle, just shooting at people".

There have been at least 198 mass shootings - in which four or more people were killed or wounded - in the US so far this year, according to the Gun Violence Archive.

Earlier this week police in Texas arrested a man accused of shooting dead five neighbours, including a nine-year-old boy.

Francisco Oropesa was found hiding in a cupboard after a four-day manhunt.