A mother and daughter from the Chicago area are among those missing in Israel.
Friends and family members fear that Judith and Natalie Raanan are being held hostage by Hamas.
They have been in the country since early September and were at the Nahal Oz kibbutz in southern Israel, close to Gaza, when the attacks began. They haven't been heard from since.
Rabbi Meir Hecht said that Judith, 59, is a member of his congregation at Chabad of Evanston, north of Chicago.
"Judith is a warm, kind, generous, giving person," he told the OceanNewsUK. "She's always here to help others in the community and does whatever she can to be a friend and available to assist people in any way possible."
"We are terribly distressed over this news and we're all praying for the safety of both Judith and Natalie and of course of all the people of Israel, all the hostages and all the wounded," he said.
Natalie Raanan, 19, recently graduated from nearby Deerfield High School.
In a statement the high school said it had no further information but that counselling services were available to students.
"Sadly, we know that many students and families have been impacted by the tragic events in Israel," said Kathryn Anderson, the school's principal.
President Joe Biden said Tuesday that at least 14 American citizens have been killed by Hamas and that American citizens were among those held hostage.
"There's still so many families desperately waiting to hear the fate of their loved ones," he said.
Speaking to a crowd at a synagogue in Glencoe, another Chicago suburb, on Tuesday, Illinois Governor JB Pritzker said he had put police across the state on alert for threats against synagogues or Jewish institutions. He added that the FBI and local law enforcement had not detected any immediate threats.
Around 4% of the total Jewish population in the United States lives in the Chicago area, according to Brandeis University's American Jewish Population Project.
"There are many peace-loving Palestinians and we must honour them, but Hamas is a terrorist organization, an army of murderers backed by Iran," Mr Pritzker said. "The images and videos of the invasion, the slaughtering of Israeli families, and the kidnappings of loved ones is sickening."
