RAMALLAH, West Bank/BEIRUT - Israeli forces unleashed an aerial and ground blitz against Hamas in Gaza after a cross-border rampage by the enclave's ruling Islamist group on Oct. 7. At least 17,487 Palestinians have been killed since then, according to Gaza Health Ministry figures, while 1,200 people were killed in the Hamas incursion into Israel, according to Israeli tallies.

Aid agencies warn that a humanitarian disaster in Gaza is worsening by the hour with most of its 2.3 million people homeless and trapped in a tiny, embattled coastal enclave, with little food, water, medical care, fuel or secure shelter.

With basic infrastructure devastated, phone and internet services frequently disrupted, and a number of health statisticians having been killed or gone missing, there is increasing concern that Gaza health authorities will be unable to continue keeping an accurate count of the casualty toll.

HOW HAVE CASUALTY TOLLS BEEN COMPILED SO FAR?

In the first six weeks of the war, hospital morgues across Gaza sent figures to the health ministry's main collection centre at Al Shifa Hospital. Officials used Excel sheets to keep track of names, ages and ID card numbers of the dead and transmitted these to the Palestinian Health Ministry in Ramallah, part of the Palestinian Authority (PA) that exercises limited self-rule in the Israeli-occupied West Bank.

But Omar Hussein Ali, director of the Ramallah ministry's emergency operations centre, said that of the four officials who ran the Shifa data centre, one died in an air strike that hit the hospital while the other three went missing when Israeli forces seized the premises as an alleged Hamas hideout.

"The kind of casualty recording required to understand what's going on is getting harder. Information infrastructure, the health systems that existed, are being systematically destroyed," said Hamit Dardagan of Iraq Body Count, set up during the U.S.-led invasion and occupation of Iraq.

Dardagan's organisation has also been trying to keep track of Gaza casualties, using the health ministry data and monitoring social media and other media reports of deaths.

Since a one-week truce collapsed on Dec. 1, casualty updates that had generally been issued daily have become irregular. The last update from Gaza's health ministry came on Friday from spokesperson Ashraf Al-Qidra, raising the death toll to 17,487. Early on Saturday it said another 71 dead and 160 injured people had arrived at Al Aqsa hospital in the past 24 hours.

Al-Qidra, who had been out of contact for almost two days earlier in the week, spoke at a brief news conference from Nasser Hospital in south Gaza, where all week medics have tried to treat badly wounded young children, women and men with limited supplies and often under aerial bombardment as the Israeli offensive continues.

Palestinian Health Minister Mai al-Kaila said on Tuesday Gaza's health services were in a "disastrous" state, with over 250 staff killed and at least 30 arrested by Israeli forces.

ARE THE PUBLISHED CASUALTY NUMBERS COMPREHENSIVE?

No, experts told Reuters.

"Our monitoring suggests that the numbers provided by the Ministry of Health may be under-reporting as they do not include fatalities who did not reach hospitals or may be lost under the rubble," the U.N. human rights office spokesperson said.

"It is a logical assumption that the numbers being reported are underestimated, are low," said Nathaniel Raymond, Executive Director of the Humanitarian Research Lab at the Yale School of Public Health, who has worked on death counts in armed conflict and natural disasters for more than 20 years.

The PA's Oct. 26 report said at least 1,000 bodies could not be recovered or brought to morgues, citing families interviewed by its Gaza staff - a clear and plausible example of the impact of war "on data capture and reporting", the Lancet article read.

The number of bodies feared buried under the rubble now reaches into the thousands and much of the Gaza civil defence force's digging equipment has been destroyed in air strikes, the PA's health minister al-Kaila said on Tuesday.

HOW CREDIBLE ARE THE CASUALTY FIGURES TO DATE?

Pre-war Gaza had robust population statistics - from a 2017 census and more recent U.N. surveys - and well-functioning health information systems better than in most Middle East countries, public health experts told Reuters.

Oona Campbell, professor at the London School of Hygiene and Tropical Medicine, said Palestinian health authorities have longstanding credibility with their methods of maintaining baseline statistics and tracking deaths in general, not just during times of war. U.N. agencies rely on them.

"Palestinian data collection capabilities are professional and many ministry staff have been trained in the United States. They work hard to ensure statistical fidelity," said Yale University's Raymond.

On Oct. 26 the PA Health Ministry published a 212-page report with the names, ages and ID numbers of 7,028 Palestinians it recorded as dead from air strikes - after U.S. President Joe Biden cast doubt on the casualty figures.

Campbell and two other academics analysed the data for a Lancet medical journal report on Nov. 26 and concluded there was no obvious reason to doubt their validity. "We consider it implausible that these patterns (of mortality rates) would arise from data fabrication," the researchers wrote.

The PA Health Ministry has not issued a similar detailed report since, a reflection of fading communications with Gaza.

There were questions about casualty figures reported from an Oct. 17 explosion at al-Ahli al-Arabi Baptist hospital in Gaza City. Gaza's health ministry said 471 people were killed in the blast, which Palestinians said was caused by an Israeli air strike but which Israel and outside groups including Human Rights Watch said was probably caused by a misfiring Palestinian rocket. An unclassified U.S. intelligence report estimated the death toll "at the low end of the 100 to 300 spectrum".

But overall, international bodies have attested to the reliability of Palestinian casualty figures.

WHAT DOES ISRAEL SAY?

A senior Israeli official told journalists on Monday that around a third of those killed in Gaza so far were enemy combatants, estimating their number at less than 10,000 but more than 5,000, without detailing how the estimate was reached. The official said the total count of some 15,000 dead as of Monday given by Palestinian authorities, who do not break down the count between civilians and combatants, is "more or less" right.

Rights groups and researchers say the high civilian toll arises from heavy weapons used - including so-called "bunker buster" bombs aimed at destroying Hamas' strategic tunnel network - and strikes on residential districts where Israel says Hamas has hidden militant bases, rocket launch pads and weaponry within and under apartment blocks and hospitals.

WHAT'S THE BREAKDOWN OF CHILDREN VERSUS ADULTS KILLED?

The United Nations, as well as Israeli and Palestinian law, define a child as someone under the age of 18, though some Hamas militants are believed to be teenagers.

The PA health ministry said on Tuesday about 70% of Gaza's dead were women and children under 18, but it has released no breakdown of age categories since its Oct. 26 report.

The Lancet article said the ministry report's data showed that 11.5% of the deaths recorded between Oct. 7-26 were children between ages 0 and 4, 11.5% between ages 5 and 9, 10.7% between 10 and 14 and 9.1% between 15 and 19.

"There was a distinct peak among men aged 30–34 years, possibly reflecting combatant or civilian exposures (e.g., first responders at bomb sites, journalists, and people going out to seek water and food for their families)," it said.

COULD DEATH TOLLS NOW BECOME A CASUALTY OF WAR?

The new phase of Israel's offensive, expanding into Gaza's southern half from Dec. 1, has further diminished the scope for collecting reliable death toll data, Richard Peeperkorn, the World Health Organization's Gaza envoy, said on Tuesday.

"As we all know normally we get (data) from the Ministry of Health, and already for a number of days it is much more based on estimates, it's much more difficult for them," he said.

Experts said the fact that it was becoming near impossible for a previously efficient cohort of health technocrats to work was another harrowing indication of the toll of the war.

"It's a terrible sign when you get to a point, like where Sudan is, where you don't even have a death registry. That in itself shows to us aid workers that this is a worst-case scenario," Yale University's Raymond said.

(Reporting by Ali Sawafta in Ramallah and Maggie Fick in Beirut; additional reporting by James Mackenzie in Jerusalem, Jana Choukeir in Dubai, Emma Farge in Geneva, Helen Reid in London and Adam Makary in Cairo; writing by Maggie Fick; editing by Mark Heinrich, Miral Fahmy and David Evans)