A 17-year-old boy has been charged with four counts of first-degree murder after shootings in Canada's western Saskatchewan province, police say.

The boy - who cannot be named because of his age - is also charged with seven counts of attempted murder.

Four people were killed and several injured after the shootings at La Loche Community School and another location.

Friday's attack in the remote community was "every parent's worst nightmare", Prime Minister Justin Trudeau has said.

'Pain and sorrow'

At a news conference on Saturday, Grant St Germaine from the Royal Canadian Mounted Police (RCMP) said the suspect would appear in court next week.

"The days and weeks are going to be difficult as we come to terms with the impact of what has happened in our community," he said.

"It's a sad and difficult time and no words will take away the pain and sorrow of what has happened."

The RCMP said Dayne Fontaine, 17, and his brother Drayden, 13, were shot dead in a house.

The gunmen then went to the school, killing teacher Adam Wood, 35, and 21-year-old teaching assistant Marie Janvier, the daughter of acting mayor Kevin Janvier.

The suspect was later disarmed and arrested. The motive behind the shootings is unclear.

Canada's gun laws

-To buy a gun or ammunition, Canadians must have a licence under the Firearms Act, which requires a safety course, a waiting period and background checks

-There are three classifications of guns: non-restricted - shotguns and rifles, commonly used for hunting; restricted - certain handguns and some semi-automatic long guns; and prohibited - most handguns, fully automatic firearms, converted automatics, firearms with a sawed-off barrel, and some military rifles like the AK-47

-In 2012, the government of Canada ended the Long Gun Registry, a database linking guns with their respective owners - this means gun owners no longer have to register non-restricted or non-prohibited guns

-Canada's gun-related homicide rate (0.5 per 100,000) is about seven times lower than the United States' (3.5 per 100,000). However, it is higher than many of its peers, including Japan (0.01 per 100,000) and the United Kingdom (0.06 per 100,000)

-Gun-related crime is highest in the provinces of Saskatchewan and Manitoba, which also report the highest rates of non-gun-related violent crime

Witnesses at the school described screaming and more than half a dozen shots.

The RCMP said officers were called about "an active shooter" at 13:00 local time (19:00 GMT) and detained the suspect 45 minutes later, Canada's CBC broadcaster reports.

"Words cannot express my shock and sorrow at the horrific events today in La Loche," Brad Wall, the premier of Saskatchewan, said in a statement.

About 900 students attend La Loche Community School for kindergarten through to grade 12 (the last year of high school).

"I ran outside the school," Noel Desjarlais, a Grade 10 student, told CBC.

"There was lots of screaming, there was about six, seven shots before I got outside. I believe there was more shots by the time I did get out."

La Loche is a mainly aboriginal community of about 3,000 people.

It is located nearly 900km north-west of the provincial capital, Regina.