Those negotiations have taken on added complexity now that the six hostages are confirmed to be dead, the two senior administration officials said. The painstaking deliberations over which hostages will be released in exchange for specific Palestinian prisoners must now be renegotiated. Some Middle East experts said that the fewer living hostages there are, the less pressure there will be on Netanyahu to come to a deal — U.S. officials fear the number of living hostages may number only in the dozens.

The senior administration official said Hamas’s decision to execute the hostages with negotiations under ways “calls into question” Hamas’s seriousness in the talks. Hamas has proved obstinate at various points in the talks, even as Israeli officials have made concessions, the official said.