New York (AFP) - New York Police Chief Bill Bratton has ordered desk officers onto street patrols to combat an eight percent spike in violence this year, police said Thursday.

A leaked letter from Bratton to his deputies called for the deployment of 400 extra uniformed officers during the 90-day program. No numbers have been released officially.

The New York Times said 400 was the highest number assigned to short-term street patrols in recent decades.

"This program is still in the process of being assessed as to the actual number of personnel to be temporarily assigned and the units from which they will be reassigned, as well as the areas where they may be deployed," a police spokesman told AFP.

Shootings in New York rose 8.1 percent in the first six months of the year to 521, compared to 482 in the same period last year, New York police department said.

The number of murders however fell, from 152 to 134.

Bratton's email called for the deployment of extra officers and detectives as part of a summer "violence-reduction plan."

It called for personnel to be redeployed from more than 20 different departments, including the counter-terrorism unit.

The Patrolmen's Benevolent Association, a police labor union, said it was an "unambiguous admission" that there are not enough officers "to curb gun violence, control crime and keep the city safe."

"Hiring more police officers is critical to addressing this problem and it must begin immediately," said PBA president Patrick Lynch in a statement.

There are currently 35,000 police officers in New York. The city has a population of more than eight million.