Police said besides the victim, his 4-year-old brother also sustained injuries all over his body.

A 7-year-old boy was left with a cracked skull and suffered critical head injuries after allegedly being assaulted by his mother's partner in Kerala.

The boy who is battling for life is believed to have been smashed to ground and hit on the head with a stick by the accused on Thursday while the child was trying to protect his younger brother from being beaten up by the man, police said.

Though the boy underwent emergency surgery at a private medical college hospital at Kolenchery in Ernakulam district, his condition continues to be serious, hospital sources said.

"His life is sustained completely on ventilator support now. The next 48 hours is critical... As per the latest scan report, the blood circulation to the brain seems to have stopped," a doctor said.

"He sustained skull fracture, internal bleeding and lung injury. The child also had cuts and bruises all over the body," he said.

The incident came to light after police and Child Helpline authorities received information about the child's worsening health condition from a hospital, where he was initially taken.

The boy was later shifted to the hospital in Ernakulam for emergency surgery.

Police said besides the victim, a second standard student, his 4-year-old brother also sustained injuries all over his body.

The statement of the younger sibling has been recorded and details will be known only after detailed interrogation of the mother and her partner, a 36-year-old man hailing from Thiruvananthapuram, they said.

"The accused is under our surveillance. His arrest will be recorded soon," a police official told PTI.

Various sections of IPC, including 303 (attempt to murder), 323 (punishment for voluntarily causing hurt), 325 (voluntarily causing grievous hurt) and Section 75 of the Juvenlie Justice Act have been slapped against him, the official added.

Meanwhile, Chief Minister Pinarayi Vijayan on Friday sought an immediate report from the Idukki district authorities on the incident.

He also directed the hospital authorities to ensure all possible expert treatment to the boy.

Health Minister KK Shylaja said the government will bear the treatment expenses of the child.

The health and social justice departments will take over the protection of the boy and his younger brother, who is reportedly with some relatives now.

"Stringent action will be taken against those engaged in cruelty to children. The covering up of assaults against children is also a grave crime," the minister said in a statement.