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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.
"We continue to monitor the situation closely and remain in touch with our Israeli partners, particularly the local authorities," they added.
The identities of those killed have not been announced.
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.
A US State Department spokesman, Matthew Miller, also said there were a number of missing Americans. "We are working with our Israeli partners to determine their whereabouts," he said.
Among them 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."
More on Israel-Gaza attacks
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.
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.