The U.N. children's agency said Sunday that more than 13,000 children have been killed in Gaza in Israel's offensive, adding many kids were suffering from severe malnutrition and did not "even have the energy to cry."

UNICEF Executive Director Catherine Russell did not provide a source for the child fatality figure during an interview with CBS News.

When asked if Russell was referring to the agency's own estimate or was basing the figure on reporting from Hamas-governed Gaza, a UNICEF spokesperson pointed to a press statement by the U.N. children's agency that attributed the figure to Gaza's health ministry, a faction of the Hamas terrorists. 

"Thousands more have been injured or we can't even determine where they are. They may be stuck under rubble ... We haven't seen that rate of death among children in almost any other conflict in the world," Russell told CBS News' "Face the Nation" program.

"I've been in wards of children who are suffering from severe anemia malnutrition, the whole ward is absolutely quiet. Because the children, the babies ... don't even have the energy to cry."

Russell said there were "very great bureaucratic challenges" moving trucks into Gaza for aid and assistance.

A March 14 infographic from OCHA, the U.N. humanitarian office, cites the Gaza government media office as saying that over 13,000 children and at least 9,000 women have been killed in Gaza since Oct. 7. U.N. agencies have relied on Hamas for casualty information during the war without authenticating the data. 

International criticism has mounted on Israel due to the death toll of the war, the starvation crisis in Gaza, and allegations of blocking aid deliveries into the enclave.

A U.N. expert said earlier this month that Israel was destroying Gaza's food system as part of a broader "starvation campaign." Israel rejected the accusation.

Israel's military assault on Gaza has displaced nearly its entire 2.3 million-person population, caused a starvation crisis, flattened most of the enclave, and killed over 31,000 people, according to Gaza's health ministry. It has also led to accusations of genocide being probed in the World Court.

Israel denies the genocide charges and says it is acting in self defense after the Oct. 7 attack on Israel from Hamas that killed some 1,200, according to Israeli tallies, and took scores of hostages.