Serbia's Interior Ministry warned gun owners on Thursday to keep their weapons empty and locked in gun cabinets or safes.

The warning comes a day after a 13-year-old boy opened fire on his classmates, killing eight children and a security guard at a primary school in the capital Belgrade. A teacher and six pupils were also wounded.

"The Interior Ministry is appealing to all gun owners to store their guns with care, locked up in safes or closets so they are out of reach of others, particularly children," it said in a statement.

Police will do spot checks at registered gun owners' homes to ensure they keep weapons safe and those who are found to be negligent could be charged.

It is part of a host of gun control measures announced by President Aleksandar Vucic hours after Wednesday's shooting.

He also announced a moratorium on new gun licenses other than for hunting and said there would be a revision of existing permits and enhanced surveillance of shooting ranges.

Toxicology tests negative

The suspected teen shooter had visited shooting ranges with his father and used his pistols in the attack, police said.

The boy was admitted to a psychiatric institution for an evaluation, and a toxicology test returned negative, authorities said on Thursday. Under Serbian law he is too young to be held criminally liable.

Police said the teenager had planned the attack for a month, drew sketches of classrooms and made lists of children he planned to kill.

Meanwhile, two of the children who were wounded but survived the shooting were still in critical condition. The remaining children and the teacher were in stable condition in hospital.

Three days of mourning

The Vladislav Ribnikar primary school where the shooting remained sealed off on Thursday. Heaps of flowers, small teddy bears, a gray and pink toy elephant and a girl's ballet shoes hung on the fence.

To help people deal with the tragedy, authorities announced they were setting up a helpline. Hundreds answered a call to donate blood for the wounded victims.

A three-day mourning period to remember the victims will begin Friday morning.

lo/rs (AP, AFP, Reuters)