Former "Eastenders" actress Brooke Kinsella made a tearful appeal on Wednesday for witnesses to her brother's stabbing to come forward, saying their help would be vital in catching his killers.

Ben Kinsella, 16, was attacked after a fight at the Shillibeers pub and nightclub in north London in the early hours of Sunday.

The youngster, the 17th teenager to die violently in the capital this year, was chased down a street and stabbed to death by up to four youths. A post mortem revealed he had suffered multiple stab wounds.

Brooke Kinsella said she was grateful to the public for their help so far but begged others, particularly those in the road at the time of the attack who might have information, to speak to detectives.

"Unfortunately the police have told us they know of others who can provide crucial evidence but they have chosen not to tell police what they know," she told a news conference.

"I have been told that Ben's murder will be really, really difficult to solve unless those witnesses come forward and help. On behalf of my family and my brother Ben please ring up and speak to the officers."

WITNESSES SCARED

She said she recognised some people might be scared to put themselves forward.

"I completely understand that, everybody is scared for their lives," she said, adding that police had guaranteed protection for witnesses.

"As far as I'm concerned there's no issue with protecting witnesses," said Detective Chief Inspector John Macdonald.

"In this inquiry already we have dealt with witnesses who have got concerns for their safety. People have provided important information."

He also called on parents whose children were in the pub on the night to "dig a bit harder" and find out what they knew.

"There's no doubt in my mind some of those kids will have said to their parents, I was there but I didn't see anything. Quite frankly I don't necessarily believe all of that."

Detectives said there had been a "minor" disturbance in the pub earlier in the night, but that Kinsella had not been involved in any fight. Nor was the attack gang-related, he said.

"It appears to be a dispute over nothing inside a pub," Macdonald said. "He's a child who has been killed for no reason."

Three teenagers, two aged 18 and a 19-year-old, are currently in police custody in connection with the murder.

A fourth suspect, aged 18, has been released on police bail while two 16-year-olds, friends of Kinsella who were arrested after the incident, have been freed and will face no further action.

On Tuesday, hundreds of people took part in a protest against the rising tide of teenage violence led by the Kinsella's family and friends, marching through north London to the place where he was stabbed.

On Wednesday, London police said they had arrested more than 1,200 and seized 528 knives during a six-week crackdown sparked by the growing number of teenage stabbings.

"The dreadful murder of Ben Kinsella shows that the problem of knife crime is still with us," said London Police Chief Ian Blair.