The White House says that a three-year-old American whose parents were killed by Hamas is among the hostages in Gaza.
President Joe Biden raised the issue of hostages, including the toddler, with a call with Amir Sheikh Tamim Bin Hamad al-Thani of Qatar on Sunday.
It comes as the White House says “urgent” efforts to free hostages taken after the 7 October attack by Hamas are continuing.
The US says there are 10 Americans still unaccounted for since the attack.
According to a readout of the call between Mr Biden and Sheikh Tamim released by the White House, the president “condemned unequivocally the holding of hostages by Hamas, including many young children, one of whom is a three-year old American citizen toddler”.
Both leaders said “all hostages must be released without further delay”.
Qatar was instrumental in efforts to release two US hostages who were abducted by Hamas’s deadly raid on Israel last month and has been central to continuing efforts.
Mother and daughter Judith and Natalie Raanan, of Chicago, were released in late October, two of four hostages who have been freed. A fifth was rescued by Israeli forces.
Israel Defense Forces have previously said the hostages include 20 children and between 10 and 20 people over the age of 60.
On Sunday, US national security adviser Jake Sullivan told CBS politics programme Face the Nation that active negotiations to release hostages were “under way between Israel and Qatar, who is communicating with Hamas, and the United States is involved in those discussions, very much involved in those discussions”.
“We are actively working to ensure the safe return of every American being held hostage and every other person being held hostage by Hamas,” he said.
President Biden’s top Mideast adviser Brett McGurk is travelling to Israel and other countries in the region, including Saudi Arabia and Qatar, this week to work on the release of US hostages.
Mr Sullivan told ABC News that nine US citizens and one permanent resident are currently missing.
He is expected to meet the family members of the American hostages this week.
CBS reported on Sunday that two US officials told the network that one of the main sticking points in the current negotiations is the fact that Hamas has yet to present a list of names of those it is holding or would be able to free. Other groups in the region, including Islamic Jihad, also have captives.
Israel began striking Gaza after the Hamas attacks a month ago, which saw 1,200 people killed and more than 240 taken hostage.
The Hamas-run health ministry says more than 11,000 people have been killed in Gaza since – of whom more than 4,500 were children.
Mr Biden and Sheikh Tamim also discussed the need to protect civilians and the delivery of humanitarian assistance to the territory.