A gunman entered a nursing home in northwestern Croatia on Monday shooting dead five people, including his mother, and injuring six others, a government minister said.

One of the wounded later died in hospital taking the death toll to six, while four remain in critical condition.

One of the victims was a nursing home employee, according to Marin Piletic, minister for labour, pension, family and social policy.

"According to the information we have the mother of the killer had been in the nursing home for 10 years," Piletic said.

Authorities have given no motive for the attack.

Croatian media reported that the gunman, born in 1973, is a war veteran.

"We are shocked," Prime Minister Andrej Plenkovic told reporters in Split.

"This is really a monstrous act of the murder of a group of people, of the mother and other very old people who happened to be there.... We condemn this crime."

The killer fled the scene, but was later arrested by the police, N1 news portal reported. (Reuters)