Already, the fragile deal has come under considerable stress, and it could collapse in the weeks to come. Yet for the time being, the fighting has stopped in both Gaza and Lebanon, and hostages have begun to come home.

The visit by the envoy, Steven Witkoff, was set to take place as Israeli and Hamas officials prepared for a new round of negotiations aimed at cementing the current cease-fire in Gaza and allowing more exchanges of hostages and Palestinian prisoners.

An Israeli government spokesman, David Mencer, said on Monday that Hamas had provided a list indicating that 25 of the 33 hostages were alive and that eight had been killed. “The list from Hamas matches Israel’s intelligence,” Mr. Mencer added.

Eight of the remaining hostages set to be released by Hamas in the first phase of a ceasefire agreement with Israel are dead, according to an Israeli government spokesperson.

Tens of thousands of Palestinians streamed into the most heavily destroyed part of the Gaza Strip on Monday as Israel lifted its closure.