At least 14 Americans have been killed in the Hamas terrorist attacks in Israel, President Joe Biden said Tuesday in an address to the American people.

He condemned Hamas' incursion as an "act of sheer evil."

Biden confirmed that American citizens are among those being held by Hamas.

He said he's directed his "team to share intelligence and deploy additional experts from across the United States government to consult with and advise Israeli counterparts on hostage recovery efforts."

The families of 155 Israeli soldiers have received death notices so far, the Israel Defense Forces said.

The U.S. has started reaching out to the families of Americans missing in Israel as part of its efforts to identify the U.S. citizens who may be among the hostages taken to Gaza by Hamas, according to an official.

But due to the dynamic situation and lack of visibility inside Gaza, the U.S. still can't say how many may be detained.

There's also a level of reluctance from U.S. officials to shine a spotlight on any Americans among the hostages for fear that they might be singled out by their Hamas captors.

Other sources said American officials are working the phones with allies in the Middle East who have leverage over Hamas, encouraging them to pressure the militants to free the hostages.

-ABC News' Shannon Crawford

United States officials are discussing whether a second U.S. aircraft carrier should be deployed to the eastern Mediterranean in the wake of the Israel-Hamas crisis, two U.S. officials confirmed.

The USS Dwight Eisenhower was previously scheduled to depart Norfolk, Virginia, later this week for the Middle East, via the Mediterranean Sea.

Officials must decide whether it will now remain in the Mediterranean and head to the east to join the USS Gerald R. Ford aircraft carrier -- which was deployed on Sunday -- or whether it will relieve the Ford.

Either way, the Eisenhower will soon be in the Mediterranean for the previously scheduled transit and a training rotation.

-ABC News' Luis Martinez

The U.S. is sending a carrier strike group to the eastern Mediterranean to bolster deterrence in the region and "will be rapidly providing the Israel Defense Forces with additional equipment and resources, including munitions," according to a statement from Defense Secretary Lloyd Austin on Sunday.

"Today, in response to this Hamas attack on Israel, and following detailed discussions with President Biden, I have directed several steps to strengthen Department of Defense posture in the region to bolster regional deterrence efforts," he said.

The U.S. materiel support will begin shipping today and will arrive in the coming days, according to Austin.

The strike group will include the USS Gerald R. Ford aircraft carrier, the guided-missile cruiser USS Normandy and four guided-missile destroyers -- the USS Thomas Hudner, USS Ramage, USS Carney and USS Roosevelt, Austin said.

The U.S. will also boost its F-35, F-15, F-16 and A-10 fighter squadrons in the region, according to the defense secretary.

"The U.S. maintains ready forces globally to further reinforce this deterrence posture if required," he said.

-ABC News' Matt Seyler