Prime Minister Gordon Brown pledged to introduce new measures to tackle knife crime on Friday, after four fatal stabbings in a single day in London brought the total killed in the capital this year above 50.

Police acknowledged that youth knife crime was a dire problem, but denied London was in the grips of a stabbing "epidemic", feared by politicians and the news media.

"The terrible stabbings that have occurred this week are shocking and tragic, and my thoughts are with the families and friends of those who have been hurt or killed in these attacks," Brown said in a statement, adding his government would announce new measures next week.

One of the latest victims to die in four separate incidents in London on Thursday was aged 19 - the 20th teenager to die violently in the capital this year. Before Thursday's killing, 49 people had died of knife wounds this year in London.

Violence in the London has become an international issue after the frenzied stabbings of two French students in London last week, an incident police said was one of the grisliest cases they had seen.

Police say tackling knife crime has overtaken terrorism as their number one priority. They have set up a 75-strong team to target those carrying weapons.

"I wouldn't describe it as an epidemic," a police spokesman said after the latest killings.

"There is an issue with knives and that is why we have launched Operation Blunt 2," he added, referring to a six-week campaign in which officers have searched 27,000 people, arrested more than 1,200 and seized 500 knives in London.

Metropolitan chief Ian Blair said detectives had made arrests in three of the four murders.

"I want to reassure the public that the MPS is doing everything possible both in terms of thoroughly investigating each case and in continuing to carry out proactive operations to get knives off the streets," he said.

Authorities have warned teenagers that carrying a knife makes them more at risk, not safer, because of the danger the weapon will be used against them.

Police say that London is still safe for a city of its size, and that the overall murder rate has not risen substantially. In the year ending in May this year there were 162 murders. There were 155 the previous year, and 210 five years ago.

New York, a city of similar size and wealth, had 496 murders in 2007, and as many as 2,262 in 1990, before crime there fell.

One suspect has been charged and three others arrested in connection with the killing of the French students.