U.S. is verifying reports that “Several” Americans are dead or missing in Israel

TEL AVIV, Israel 

Israeli soldiers battled Hamas fighters in the streets of southern Israel on Sunday and launched retaliation strikes that leveled buildings in Gaza, while in northern Israel a brief exchange of strikes with Lebanon's Hezbollah militant group raised fears of a broader conflict.

There was still some fighting underway more than 24 hours after an unprecedented surprise attack from Gaza, in which Hamas militants, backed by a volley of thousands of rockets, broke through Israel's security barrier and rampaged through nearby communities. 

At least 600 people have reportedly been killed in Israel — a staggering toll on a scale the country has not experienced in decades — and more than 300 have been killed in Gaza.

The militants also took captives back into the coastal Gaza enclave, including women, children and the elderly, whom they will likely try to trade for thousands of Palestinian prisoners held by Israel.

Secretary of State Antony Blinken told CNN's State of the Union that the U.S. is working to verify reports that “several” Americans were killed or are missing. 

The high death toll, multiple captives and slow response to the onslaught pointed to a major intelligence failure and undermined the long-held perception that Israel has eyes and ears everywhere in the small, densely populated territory it has controlled for decades.

Israeli Prime Minister, Benjamin Netanyahu said the country was at war and would exact a heavy price from its enemies. 

His Security Cabinet officially declared the country at war in an announcement on Sunday, saying the decision formally authorizes “the taking of significant military steps."

The implications of the announcement were not immediately clear. 

Israel has carried out major military campaigns over the past four decades in Lebanon and Gaza that it portrayed as wars, but without a formal declaration.

Yohanan Plesner, the head of the Israel Democracy Institute, a local think tank, said the decision is largely symbolic, but "demonstrates that the government thinks we are entering a more lengthy, intense and significant period of war.”

A major question now was whether Israel will launch a ground assault into Gaza, a move that in the past has brought intensified casualties. 

Netanyahu vowed that Hamas “will pay an unprecedented price.” But, he warned, “This war will take time. It will be difficult.”

Civilians paid a staggering cost for the violence on both sides. Several Israeli media outlets, citing rescue service officials, said at least 600 people were killed in Israel, including 44 soldiers, while officials in Gaza said 313 people had died in the territory. 

Some 2,000 people have been wounded on each side. An Israeli official said security forces have killed 400 militants and captured dozens more.

 Israeli TV news aired a stream of accounts from the relatives of captive or missing Israelis, who wailed and begged for assistance amid a fog of uncertainty surrounding the fate of their loved ones. 

In Gaza, residents fled homes near the border to escape Israeli strikes, fleeing deeper inside the territory after warnings in Arabic from the Israeli military.