The doctor leaned over a small boy lying on a bed in a children's hospital in the northern Ukrainian city of Kharkiv and asked: "Little Vova, how are you?"

"I am fine," came the barely audible reply. "Fine," the doctor repeated loudly, satisfied by what he heard.

Medics said Vova was hit in the head by a bullet during fighting in the first few days of Russia's invasion of Ukraine, one of several children brought into the hospital to be treated for shrapnel and bullet wounds.

Kharkiv, which has close historical and social ties to Russia, has seen some of the heaviest fighting since the war began on Feb. 24.

Ukrainian regional police official Serhiy Bolvinov said on Tuesday that 170 civilians had been killed across Kharkiv region since the start of Russia's invasion, including five children.

Russia denies targeting civilians and says its forces are engaged in a "special military operation" in Ukraine to destroy its neighbour's military capabilities and remove what it regards as dangerous nationalists in Kyiv.

Ukraine and its allies call Russia's actions a brutal invasion that has killed hundreds of civilians. Apartment blocks in Kharkiv and elsewhere have been reduced to rubble, towns have been evacuated and some 2 million people have fled the country.

At the pediatric neurosurgery centre of the city's main accident and emergency hospital, eight-year-old Dima Kasyanov lay unconscious on an intravenous drip with a tube coming out of his mouth.

His father, Sergiy, showed Reuters an X-ray image on his mobile phone that he said was of a piece of shrapnel from an incoming shell that had lodged at the base of Dima's skull.

"The shrapnel entered through the jaw and lodged itself in the neck, at the top of the vertebral column. It was yesterday. He is eight," he told Reuters. The child's mother sobbed quietly next to him.

Sergiy Kasyanov said the shell struck the family's apartment and everyone ran for safety. He said his mother had also been wounded and was in intensive care with broken ribs and damage to her spine.

"But the child, he suffered the worst injury."

Oleksandr Dikhnovskiy, the doctor who heads the centre, said the unit had operated on four children with shrapnel or bullet wounds so far.

"Sadly one little girl died yesterday. She was admitted on the first night of the ... invasion of our country."

He said Dima's treatment would be complex.

"The surgery is very complicated from a technical point of view and we are now stabilising him to be able to conduct the surgery."

In the surgery theatre, a Reuters reporter heard the rumble of a distant explosion.

"This is how we work every day, conducting surgery under bombardment," said one of the nurses.

(Writing by Mike Collett-White; Editing by Cynthia Osterman)