NEW YORK (AFP) - A four-year-old American boy shot and wounded a six-year-old friend in the head on Tuesday, police said, the second such accident involving children in a day.

Police in the New Jersey town of Toms River said the young boy took a .22 calibre rifle and shot his friend at a distance of about 14m.

It was unclear why the weapon discharged. "The young boys were playing," Police Chief Michael Mastronardy told a press conference.

"The four-year-old went into the house, retrieved the rifle from within the house: a .22 caliber rifle." "A shot went out, and the six-year-old was struck in the head," he said. The victim was in hospital in "serious" condition.

Meanwhile in Tennessee, another four-year-old pulled the trigger of a loaded handgun and killed the wife of the local sheriff's deputy, the gun's owner.

"Split second, we're talking about seconds for that kid to walk in that room unbeknownst to them, grab that gun and it goes off," said Wilson County Sheriff Robert Bryan, speaking on local television.

"It looks to be a tragic accident," he told USA Today. "He did not normally have small kids at his house, and his guns were locked prior to coming out."

Guns are linked to more than 31,000 deaths a year in the US, most of them self-inflicted. Only around a third of the deaths are considered homicides.