At least nine Americans have been killed in Israel after Hamas militants launched an attack on the country.
"We extend our deepest condolences to the victims and to the families of all those affected," a US spokesperson said when confirming the deaths.
The identities of those killed have not been announced.
The US also said some Americans were still unaccounted for two days after the attack was launched, and there are fears they may have been abducted.
"We are working with our Israeli partners to determine their whereabouts," US State Department spokesman Matthew Miller said on Monday.
Israel says more than 700 people have been killed and 100 kidnapped since Hamas militants launched the surprise attack in the early hours of Saturday.
In Gaza, almost 500 people have been killed following retaliatory Israeli air strikes.
Among the missing is 23-year-old American-Israeli Hersh Golberg-Polin, who was at a music festival in the desert which was stormed by militants.
His parents told the Jerusalem Post they received two messages from him, reading "I love you" and "I'm sorry."
"We just want him safe," Jonathan Polin, his father, said.
Israel's ambassador to the US, Michael Herzog, told CBS News that he understood Americans were among soldiers and civilians who were abducted in southern Israel but did not have details.
Global Affairs Canada has meanwhile confirmed at least one Canadian citizen is dead and two are missing.
Ben Mizrachi, from British Columbia, is among them, CTV News reported.
Mr Mizrachi graduated from Vancouver's King David High School five years ago and had been attending an event in southern Israel when the attack occurred, the school said.
A former federal minister of justice, Irwin Cotler, wrote online that Vivian Silver, 75, a Canadian-Israeli, was "violently taken from her home in Kibbutz Be'eri" on Saturday and "is now being held captive by Hamas in Gaza".
The peace activist from Winnipeg is the long-time director of the Arab Jewish Center For Empowerment, Equality, and Co-operation, a non-profit that has promoted a shared society for Arabs and Jews throughout Israel.
Foreign nationals from various countries have been confirmed to have died in the attack. More than 10 British citizens are feared dead or missing, an official UK source told the OceanNewsUK.
On Monday, the Israel Defense Forces (IDF) declared that security forces had regained control of Gaza border communities and killed a number of militants.
A Hamas spokesman claimed that four Israelis taken hostage during Saturday's unprecedented attack were among those killed in air strikes on Sunday and Monday.
Israel's defence minister, meanwhile, said he had ordered a "complete siege" of Gaza, cutting off its supplies of power, fuel and food.
The two million Palestinians in the Hamas-ruled territory are also bracing themselves for a possible ground operation, with Israeli forces massing nearby.
