World Health Organization chief Tedros Adhanom Ghebreyesus said Friday he was "very concerned" about the growing threat of infectious diseases in the Gaza Strip.

"As people continue to be massively displaced across the south of Gaza, with some families forced to move multiple times and many sheltering in overcrowded health facilities, my WHO colleagues and I remain very concerned about the increasing threat of infectious diseases," Tedros said on X, formerly Twitter.

Israel has vowed to destroy Hamas — considered a "terrorist" group by the United States and European Union — in retaliation for the October 7 attack which left about 1,140 people dead, mostly civilians, according to an AFP tally based on Israeli figures.

Around 250 hostages were also taken during the attack, more than half of whom remain captive.

Israel's relentless aerial bombardment and ground invasion in Gaza have killed at least 21,320 people, mostly women and children, according to the health ministry in Hamas-ruled Gaza.

Tedros said that since mid-October until mid-December, people living in shelters had continued to fall sick.

He said that close to 180,000 people were suffering with upper respiratory infections, while 136,400 cases of diarrhoea have been recorded — half of those among children aged under five.

The UN health agency chief said there had been 55,400 cases of lice and scabies; 5,330 cases of chickenpox; and 42,700 cases of skin rash, including 4722 cases of impetigo.

"WHO and partners are working tirelessly to support the health authorities to increase disease surveillance and control by supplying medicines, testing kits to support prompt detection and response to infectious diseases such as hepatitis, and trying to improve access to safe water, food, hygiene and sanitation services," said Tedros.