The World Health Organization (WHO) urged a safe corridor through Poland to transfer medical aid to Ukraine, warning that the country's hospital oxygen supplies were dangerously low.

WHO chief Tedros Adhanom Ghebreyesus and Europe regional director Hans Kluge said Sunday that oxygen supplies were "nearing a very dangerous point" and that most hospitals could exhaust their reserves within the next 24 hours, putting thousands of lives at risk.

The WHO is working to deliver oxygen cylinders and liquid oxygen from regional networks, they said, adding that the supplies would need "safe transit, including via a logistics corridor through Poland."

"It is imperative to ensure that life-saving medical supplies – including oxygen – reach those who need them," they said in a joint statement as Moscow's incursion into its neighbor reached its fourth day.

The disruption caused by Russia's invasion of Ukraine has prevented trucks from transporting oxygen from plants to hospitals, which have also suffered power shortages.

Medical oxygen generator manufacturers are also facing a shortage of zeolite, a mainly imported chemical product essential for producing medical oxygen.

Concerns have grown over a looming humanitarian crisis as hundreds of thousands of Ukrainian civilians flee to neighboring countries or become internally displaced.