Witnesses who fled the scene said two pupils wearing masks and trenchcoats opened fire with automatic weapons and threw homemade bombs at Columbine High School in the suburbs of Denver, Colorado.

The BBC's Tom Carver: "Clinton stuggled to explain what provoked the carnage"

If the fatalities are confirmed, it will be the worst-ever school shooting in the United States.

At least 18 other people were injured and are being treated in hospital, according to police.

Officers have yet to remove any of the victims' bodies - which are in rooms throughout the school - because of the danger of explosives and the need to preserve evidence.

The bodies of the two suspects, who had apparently shot themselves, were found in the library. Pupils name them as Eric Harris and Dylan Klebold.

Swat teams scour the premises

The local sheriff described the incident as a "suicide mission". Explosive devices were found on or near the suspects' bodies.

As FBI agents and Swat teams made their way through the carnage, a bomb set on a time device exploded. No-one was hurt.

At least 12 other bombs were found throughout the school, said police.

Clinton: 'Our prayers are with you'

With the country in shock at the shootings, US President Bill Clinton said more would have to be done to teach children to express their anger and resolve conflicts with words not guns.

President Clinton: "We must do more to reach out to our children"

"The prayers of the American people are with you," he said to the people of Littleton.

He also said the surviving children, parents and teachers should be given counselling as soon as possible.

'Trenchcoat mafia'

Many of the pupils said the gunmen belonged to a group known as the "trenchcoat mafia", who wore long black trenchcoats, boasted about owning guns, and were alienated from the other children.

Washington Correspondent Paul Reynolds: Could have been revenge

One girl told police that she was sitting in the library when one of the gunmen, dressed in a black mask, trenchcoat and gloves, began firing shots.

"He said he would kill everyone who had been mean to him and his friends over the last year," she said.

Police question a pupil believed to be a friend of the suspects

Some reports say the gunmen were targeting racial minorities and popular athletes.

One boy reported that one of the gunmen said: "If there are any jocks (sporty people) in here, they're dead."

Pupils at the school described the suspects as "militaristic", and said one of them had lots of guns and a "hate-filled Website".

They were also said to have made a video of their guns during a video class.

'We didn't think it was real'

The gunmen are said to have targeted crowded areas, such as the canteen and library.

As they began firing at about 1130 local time (1730GMT), many pupils fled or tried to hide.

Four witnesses describe the shooting

"They didn't care who they shot. They were just shooting," a pupil told a local television station.

"We didn't think it was real and then we saw blood."

More than 200 heavily-armed police from throughout the Denver area surrounded the school.

Tom Foreman on ABC News: Teachers yelled for students to run

Children who had hidden from the gunmen were released in groups by the police search teams before joining crowds of anxious parents.

Three people believed to be friends of the suspects were arrested and were led away in handcuffs.

But a police spokeswoman said they were not thought to be directly responsible for the shootings.

Guns in school

Columbine High School - which has 1,800 pupils - is situated in the middle-class suburb of Littleton.

The incident is the latest in a series of shootings in American schools.

BBC Washington Correspondent Rob Watson says it is certain to reignite the debate as to what makes teenagers turn killers.

In the wake of the shooting in Jonesboro last year, there were calls to tighten up the US gun laws.

But it is still relatively easy for children to get hold of guns from parents and friends, and take them to school.