Malnutrition, child deaths and drug shortages -- healthcare in west Mosul is getting worse by the day as Iraqi forces press an offensive to wrest it from the Islamic State group.

"Our neighbours' son died four days ago," Abu Ahmad, a resident of Bab al-Jadid district, told AFP by phone from the jihadists' last remaining Iraqi stronghold.

"The lack of food, combined with the boy's fragile health, killed him. He was just six years old."

Abu Ahmad, who did not want to give his full name, said many residents had gone weeks without more than a single small meal a day, often just yoghurt or boiled potatoes.

Meanwhile, essential drugs are running out and civilians are unable to access healthcare.

"Daesh fighters have seized all the hospitals and only they can get treated now," an employee at Al-Jamhuri hospital said, using an Arabic acronym for IS.

Three children aged three to six had died from "malnutrition and lack of medicines at health clinics and pharmacies" and more deaths could be expected in the coming days, the official said.

"Even before the hospitals were closed, locals had to pay Daesh sums of money they couldn't afford."

Healthcare was free at the point of delivery before the group seized Iraq's second city in 2014.

- Children 'worst affected' -

Iraqi forces completed their recapture of eastern Mosul in January, reaching the east bank of the Tigris river that divides the country's second city.

On Sunday they launched an operation to oust the jihadists from Mosul's west.

The same day, the charity Save the Children warned that around 350,000 people under the age of 18 were trapped, calling for safe escape routes for civilians.

It said explosions in the narrow, densely populated streets could be "more deadly and indiscriminate" than in fighting to date.

But with Iraqi forces still kilometres (miles) from the city's western edges, hunger and the lack of healthcare are the immediate threat.

"All ages are being hit by diseases caused by malnutrition, but children are the worst affected. They're not getting food or water," Yasser Fawzi, a doctor at Al-Jamhuri hospital who escaped to the city's east, told AFP.

He said desperate residents had raided vegetable and herb stalls.

"Locals have prepared herbal mixtures to treat urgent cases of injuries and burns," said Abu Mohammad, speaking by phone from Al-Zanjili on the west bank of the Tigris.

Also speaking by phone from inside west Mosul, Abu Salem said his wife had just given birth and he could not provide food for the mother or child.

"Both are in a bad way," he said.

"As the hospitals are closed, she was forced to undergo a Caesarean section at home eight months into her pregnancy."

Umm Ali, of Al-Najjar district, said such cases were common.

"Some pregnant women are rushing to give birth before the fighting reaches the city and the situation becomes even more complicated," she said.