The Israel-Hamas conflict is taking a "horrendous toll" on families, humanitarian organizations decried this week, amid reports of the slaughter and kidnapping of children and attacks on civilian infrastructure that have killed, injured or displaced the most vulnerable.

In the days since Hamas' surprise assault on Israel, images from both regions have shown crying children running through the street and cowering in bomb shelters after airstrikes. In Gaza, the bodies of dead children killed in shelling were covered in blankets and carried by their fathers in funeral processions. In the kibbutz of Be'eri, one of the largest in Israel, more than 100 bodies of Israeli citizens were discovered on Monday, with women, children and the elderly "brutally butchered," the Israel Defense Forces said. Israeli children have also been among those reported kidnapped by Hamas terrorists.

"Nothing justifies the killing, maiming or abducting of children -- grave rights violations which UNICEF wholeheartedly condemns. Yet less than 72 hours after the outbreak of horrific violence in Israel, reports indicate that grave rights violations against children are rampant," UNICEF Executive Director Catherine Russell said in a statement on Monday. "Many children have been killed or injured, while countless others have been exposed to the violence."

According to Palestinian authorities, 900 people have been killed in Gaza so far -- including 260 children and 230 women. The number of children killed in Israel is unclear; at least 900 people have died and 2,600 others have been injured, officials said, though did not specify how many were children. Prime Minister Benjamin Netanyahu said in a speech Monday that among the "atrocities" committed by Hamas, children have been "executed with the rest of their families."

In remarks at the White House Tuesday, President Joe Biden described "Hamas' bloodthirstiness" as reminiscent of ISIS rampages -- including "stomach-churning reports of babies being killed."

At least 100 civilians and soldiers have also been taken hostage by Hamas militants, Israeli officials said. Hamas leaders on Monday threatened to begin killing hostages one by one and filming the executions if their demands are not met.

Among those abducted were 12-and-16-year-old brothers, their mother told ABC News. The woman, who asked not to be identified for security reasons, said the boys were abducted Saturday by Hamas militants who burst into a safe room at their father's home on a kibbutz near the Gaza border.

"I want the world to demand the release of those innocent civilians. I want these children and women and babies back home, and I want my children back home," the mother said. "I can't take a shower without thinking of them being held hostage in some dirty pit somewhere. I can't eat, I can't sleep. I don't think human beings treat people like this. I'm sorry. I want the world to know, to demand those hostages to be returned to their homes."

In response to the assault, Israel on Monday carried out a "complete siege," cutting off power and blocking food and water from being delivered to the Gaza Strip -- where, according to the CIA, nearly 40% of the population of 2 million is under the age of 15.

UNICEF is "extremely alarmed" about those measures, spokesperson James Elder said at a press briefing Tuesday in Geneva.

"This will add another layer of suffering to the existing catastrophe faced by families in Gaza," Elder said. "Depriving children of access to food and essential services puts their lives at risk, as do attacks on civilian areas and infrastructure."

According to UNICEF, 80% of those who live in the Gaza Strip rely on some form of humanitarian assistance.

The conflict has led to "grave humanitarian consequences," Lynn Hastings, a humanitarian coordinator for the Gaza Strip for the United Nations Office for the Coordination of Humanitarian Affairs, while calling for all parties to abide by international humanitarian law.

"Civilians, especially children, medical facilities, humanitarian personnel health workers, and journalists must be protected," Hastings said in a statement Tuesday. "Captured civilians must be released immediately and unconditionally."

UNICEF has also called on all parties to protect children from harm, in accordance with international humanitarian law.

"I remind all parties that in this war, as in all wars, it is children who suffer first and suffer most," Russell said.