Virginia Tech university, scene of the worst school shooting in US history, was on lockdown Thursday after a man possibly carrying a handgun was spotted on the campus.

So far, police have not found anyone matching the description given by three youths, who were attending a camp at the university in Blacksburg, Virginia, but were still patrolling the campus in search of the man.

"The alert is still in effect," the university's police chief Wendell Flinchum told reporters, adding no one was yet under arrest.

"The alert that went out asked people to go inside, secure themselves until further notice and that's what we're still asking people to do."

At about midday, school officials said there had been no further sightings of anything suspicious three hours after the initial report, but asked those on campus to stay inside.

"Police continue to investigate campus," the school website said. "We ask people to remain secured indoors."

The campus lockdown came four years after 32 people were gunned down and more than 20 others wounded in a horrific shooting spree by a South Korea-born student. The attacker then killed himself.

The university said in a bulletin on its website that at 9:09 am (1309 GMT), the three youths saw a "white male, six feet tall, with light brown hair outside of New Residence Hall East holding what may have been a handgun."

"They say the weapon was covered by a cloth or covering of some sort," the statement said, adding the suspect was wearing a shirt with blue and white vertical stripes, gray shorts and brown sandals.

"Officers responded immediately to the area but found no one matching the description. Police have encountered no other witnesses reporting this individual or anything suspicious," the university said.

Assistant Vice President Larry Hincker said there were probably several thousand people on campus as there were several summer camps taking place.

"There are a large number of students," Flinchum added.

"I don't want to say is it any easier given the fact that you don't have 30,000 kids roaming campus? I mean is this a little easier to get workers corralled in the building versus a bunch of students."

He said some people had already been stopped and questioned "and they were ruled out and we have not found the person matching the description that was given."

Memories are still fresh here of the 2007 massacre when Cho Seung-Hui, a 23-year-old Virginia Tech student born in South Korea, shot to death 32 fellow classmates and teachers before killing himself.

University officials came under heavy criticism at the time for failing to alert students early enough to the fact that a gunman was loose on the campus, after he first shot two students and then a few hours later unleashed a larger shooting spree in a classroom.

The university was fined $55,000 in relation to the April 16, 2007 incident for failing to alert students in time to the gunman.

But student Kelsey Heiter told CNN she had had no hesitation in attending the university despite the shooting.

"It still hits close to home, but at the same time we have received the same kind of updates that we are now... Today we have a better system. More updated," she said.

"People are receiving news a lot faster than they were then and the good thing is that I have not heard any reported incidents of anyone being hurt or injured or anything of that nature.

"I love Virginia Tech and I wouldn't trade it for anything in the world," she said, adding the" campus is very safe."