Israeli Prime Minister Benjamin Netanyahu said, in his conversation with President Joe Biden on Tuesday, he called Hamas' attack on Israel "barbarism that has not been seen since the Holocaust."

"Hundreds of people were slaughtered," including soldiers whose heads were cut off, Netanyahu said.

"Entire families were murdered in their beds and homes, women were brutally raped and murdered, [and] more than a hundred were kidnapped, including children," he said.

Netanyahu said he told Biden that "Hamas is worse than ISIS -- and that they should be treated that way."

Minister of Defense Yoav Gallant said Israeli soldiers have gained control of Israel's southern border "and are now moving on to a full-scale response."

"Anyone who decapitates citizens, or murders women and Holocaust survivors, will be eliminated. We will use all our force," Gallant said.

To the soldiers, Gallant said, "We have full confidence in you. It is thanks to the IDF that Israel exists."

The IDF said Israeli soldiers exchanged gunfire with terrorists in the Ashkelon industrial area in the last hour, killing at least three terrorists.

-ABC News' Will Gretsky

The United States is tracking about 20 Americans who are still missing in Israel, State Department spokesperson Matt Miller said Tuesday.

Miller said that doesn't necessarily mean they are being held by Hamas.

Secretary of State Antony Blinken will visit Israel this week to work with Israeli partners directly to determine "how we can continue to best support them," Miller said.

Blinken will most likely leave on Wednesday to arrive in Israel on Thursday, he said.

Biden has not spoken with the families of the 14 Americans killed in Israel, but the State Department has been in constant contact with their families, national security adviser Jake Sullivan said Tuesday.

"The president has been making this his highest priority as he receives briefings each day about what we are doing to try to determine both what’s happening with the missing and also to ensure that we can secure the bodies of those that have perished and ensure that they get returned to their loved ones," he said.

-ABC News' Shannon Crawford and Mary Bruce

American Airlines said it will suspend flights to and from Tel Aviv through Dec. 4.

Delta suspended flights to and from Tel Aviv until the end of October, while United said its flights to and from Tel Aviv are suspended "until conditions allow them to resume."

President Joe Biden said Tuesday in an address to the American people, "We stand with Israel. And we will make sure Israel has what it needs to take care of its citizens -- defend itself and respond to this attack."

Biden stressed, "Hamas does not stand for the Palestinian people's right to dignity and self-determination. Their state of purpose is the annihilation of the state of Israel on the murder of Jewish people. They use Palestinian civilians as human shields. Hamas offers nothing but terror and bloodshed, with no regard to who pays the price."

Biden said among the horrors were "parents butchered, using their bodies to try to protect their children. Stomach churning reports of being babies being killed. Entire families slain. ... Women raped, assaulted, paraded as trophies."

"There's still so many families desperately waiting to hear the fate of their loved ones, not knowing if they're alive or dead or hostages," Biden said. "Infants in their mother’s arms, grandparents in wheelchairs, Holocaust survivors abducted and held hostage. Hostages whom Hamas has now threatened to execute in violation of every code of human morality."

"It's abhorrent," Biden said, adding that Hamas' "brutality ... brings to mind the worst rampages of ISIS. This is terrorism. But sadly, for the Jewish people, it's not new."

Biden and Vice President Kamala Harris spoke with Israeli Prime Minister Benjamin Netanyahu earlier in the day.

Biden said, "We're surging additional military assistance, including ammunition and interceptors, to replenish Iron Dome. We're going to make sure that Israel does not run out of these critical assets to defend its cities and its citizens. My administration has consulted closely with Congress throughout this crisis, and when Congress returns, we're going to ask them to take urgent action to fund the national security requirements of our critical partners."

"This is a moment for the United States to come together, to grieve with those who are mourning," Biden said, adding, "There is no place for hate in America -- not against Jews, not against Muslims, not against anybody."