Police in the US state of Maine waged a major manhunt Thursday for a gunman who killed 18 people at a bowling alley and a bar in the deadliest mass shooting this year in America.

The rampage in the small northeastern town of Lewiston also left 13 people wounded, three of them in critical condition, with a wide area under lockdown as schools were closed, roadblocks erected and residents ordered to stay indoors.

Governor Janet Mills said the suspect was "considered armed and dangerous, and police advise that Maine people should not approach him under any circumstances."

"This attack strikes at the very heart of who we are and the values we hold dear," Mills told a press conference. "This is a dark day for Maine."

Police named the suspect as Robert Card -- seen in surveillance footage pointing a semi-automatic rifle with an extended clip as he walked into the bowling alley in Lewiston.

News outlets broadcast footage of people fleeing in terror from the venue after the shooting started on Wednesday evening.

One survivor told television reporters that he was 15 feet (5 meters) from the gunman when he opened fire. He thought at first it was a balloon popping.

"And as soon as I turned and saw it was not a balloon and he was holding a weapon, I just booked it down the lane and I slid basically into where the pins are and climbed up into the machine and was on top of the machines for about 10 minutes until the cops got there," he said.

Card is a certified firearms instructor and a member of the US Army Reserve, according to law enforcement.

As hundreds of law enforcement officials flooded the search zone, Lewiston police chief David St. Pierre said on Thursday that "this is an all hands on deck approach."

"We continue to work tirelessly in bringing the situation to an end," he told reporters, vowing "to locate and hold the person accountable."

President Joe Biden called Maine's governor and officials to offer federal support, and ordered flags to be lowered to half-mast at the White House and all government buildings.

"Once again, our nation is in mourning after yet another senseless and tragic mass shooting," he said.

Biden added that the gun violence that plagues the United States "is not normal, and we cannot accept it," urging lawmakers to pass a bill banning assault weapons and high-capacity magazines.

Police and rescuers reportedly arrived at the Sparetime Recreation bowling alley at about 7:15 pm in response to an active shooter, and then received reports of another shooting at the Schemengees Bar & Grille.

In camera images of 40-year-old Card at the bowling alley, he appeared calm and composed as he moved through the doorway with his rifle raised.

Another survivor, Riley Dumont, said she and her family fled to a corner of the bowling venue and hid behind tables and a bench.

"I was laying on top of my daughter. My mother was laying on top of me," Dumont told ABC News.

"It felt like it lasted a lifetime," she added. "I just remember people sobbing and crying."

Officers located a "vehicle of interest" -- a white sport utility vehicle (SUV) -- in Lisbon, a town around 12 kilometre from Lewiston.

The shooting is one of the deadliest since 2017, when a gunman opened fire on a crowded music festival in Las Vegas, killing about 60 people. (AFP)