In yet another campus shooting in the United States, a heavily-armed former student opened indiscriminate fire in a Northern Illinois University lecture hall, killing six persons and wounding 16 others, before turning the gun on himself on Valentine's Day.

This is the fourth campus shooting in a week to rock the US.

The tall white skinny assailant, dressed in black shirt and hat and dark pants, came out from behind a curtain on the stage of the lecture hall and fired over 20 shots at students in the geology class, eyewitnesses and officials said.

The gunman, whom the authorities did not immediately identify, died of a self-inflicted gunshot wound, John G Peters, the president of Northern Illinois University, told media persons.

The gunman's body was found on the stage of the lecture hall along with three weapons -- two handguns and the shotgun, with ammunition still left in both handguns. Gun magazines were found 'all over the floor,' Donald Grady, the NIU police chief, said.

"This thing started and ended within a matter of seconds," he said, adding that there was no apparent motive ascertained as of now.

Four of the dead were women. Peters said four, including the assailant, died at the scene and the other two died at the hospital, adding all the victims were students.

Shots rang out shortly after 3 pm Central Time inside Cole Hall of the 113-year-old university where 25,000 students, including, 862 foreigners, are enrolled.

"He just kicked the door open, just started shooting," Kevin McEnery, who was in the class at the time, said, adding, "All I really heard was just people screaming, yelling 'get out.'... Close to 30 shots were fired."

Security around campus was increased in December when police found threats scrawled on a campus bathroom wall that included racial slurs and references to last April's Virginia Tech killings that killed 32 people, CNN reported.

One of the threats said 'things will change most hastily' in the final days of the semester.

Peters said the motive for the 'senseless tragedy' was unknown, but authorities had 'no reason to believe' it was related to threats found in December.

Illinois Governor Rod Blagojevich has declared a state of emergency.

Desiree Smith said she saw fellow students fall down around her as the gunman opened fire. She tried to crawl away thinking she was going to die, then wondered if she should play dead before getting up to run out of the classroom.

Peters said the gunman was enrolled as a graduate student at NIU in the spring of 2007. There are about 162 registered students in the class that met in the large lecture hall.

Rosie Moroni, a student at the school, told CNN that she heard shots coming from the classroom she had intended to enter.

"Then people ran out the doors yelling, 'He has got a gun, call 9-1-1," she said, adding, "It was complete chaos... it is very scary here right now."

Most of the injuries are head and chest gunshot wounds. Events and classes were cancelled until further notice and counselling centres set up.