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US President Joe Biden said on Tuesday that Americans had been confirmed to be among the hostages taken by Hamas after the militants' surprise attack on Israel.
“We now know that American citizens are among those being held by Hamas,” Mr Biden said at the White House.
“I’m directing my team to share intelligence and deploy additional experts from across the United States government to consult with and advise Israeli counterparts, hostage recovery and recovery efforts.”
Mr Biden's address came after he spoke by phone with Prime Minister Benjamin Netanyahu about the situation in Israel.
The US military has sent an aircraft carrier strike group to the Eastern Mediterranean so the Pentagon now has expert personnel close by.
There has been no official indication so far that the Pentagon wants to send US troops into Israel to assist with hostage recovery operations.
In an emotional address, Mr Biden referenced “stomach-turning” reports of violence and likened the Hamas attacks to the worst excesses of ISIS.
"You know there are moments in this life, I mean this literally, when pure, unadulterated evil is unleashed on this world," he said. "People of Israel lived through one such moment this weekend."
Mr Biden said 14 Americans were among the hundreds of people killed in the assault.
“The brutality of Hamas, this blood-thirstiness, brings to mind the worst rampages of ISIS,” he said. “This is terrorism.”
Mr Biden also warned other groups or countries seeking to take advantage of Israel after the attacks.
“Don’t,” he said.
White House National Security Adviser Jake Sullivan said another 20 Americans were missing, although it was not clear whether they were all hostages.
"We will work hour by hour, both to determine whether we can account for any of those Americans, or to confirm exactly the number of Americans being held hostage," Mr Sullivan said.
He said the US would be sending Israel additional Iron Dome interceptors and ammunition, and looking into ways to "augment the nation's air defence capabilities".
Before the Hamas attacks, Mr Sullivan was closely involved in talks that the Biden administration had hoped would lead to a landmark normalisation deal between Israel and Saudi Arabia.
Those conversations are "not on hold" but dealing with the situation in Israel is now the priority for the US.
"Obviously over the last four days, all of our attention with respect to our support for Israel has been focused squarely on this conflict, on this heinous brutal attack, and on helping Israel be able to defend itself," Mr Sullivan said.
The State Department said that Secretary of State Antony Blinken would travel to Israel in the coming days to deliver a message of solidarity and support.
Mr Blinken will "talk about what additional resources we can give them", State Department spokesman Matthew Miller said.