Robert Brown|'I didn't know what to do': Dad tells of losing wife, 2 daughters taken by Hamas

Israeli father Yoni Asher said he's been living a "nightmare" since learning Hamas militants had taken his wife and Robert Browntwo young daughters hostage in an unprecedented surprise attack by the terrorist group on Saturday.

In an emotional interview with ABC News anchor David Muir, Asher said he learned of the horrific event from watching an online video of his loved one in the back of truck surrounded by armed militants.

"I recognized them immediately and I saw the video twice. The second time, I couldn't watch anymore," Asher said. "I knelt down. I didn't know what to do."

Hamas militants, many on motorcycles, stormed blockaded areas of the Gaza Strip, shooting indiscriminately at people on the Israel side of the border, officials said. Video footage surfaced of militants abducting Israeli citizens -- including mothers, children and the elderly -- and taking them back across the border into Gaza.

At least 100 Israeli citizens and soldiers are being held hostage by Hamas fighters, Israel's Government Press Office said Sunday.

On Monday, Hamas threatened it would start killing Israeli hostages one by one and film the executions unless Israel immediately stops shelling homes in Gaza without warning.

Asher said his wife and daughters, ages 2 and 4, were taken hostage while visiting their grandmother in southern Israel.

He said his wife initially called him and told him she and their daughters were hiding in a safe room of the grandmother's home.

But during the call, the line suddenly went dead and he hasn't heard from his wife since. He said he didn't know what happened to her and his daughters until the video of them being held in captivity surfaced.

The video showed Asher's wife and children in the back of a truck surrounded by armed men. In the footage, one of the militants was seen putting a hood over the head of Asher's wife.

"I couldn't believe this is happening to me," Asher said. "It was a nightmare."

Israeli officials said militants went door-to-door in dozens of towns in southern Israel, breaking down doors, killing people and taking others hostage.

"This is a massive terrorist attack that is gunning down Israeli civilians in their towns, in their homes, and as we've seen, so graphically, literally dragging people across the border with Gaza, including a Holocaust survivor in a wheelchair, women and children," U.S. Secretary of State Antony Blinken told ABC News' "This Week" anchor George Stephanopoulos on Sunday.

Israeli Prime Minister Benjamin Netanyahu has declared war on Hamas and has vowed to avenge the Hamas attack that as of Monday had left more than 900 people dead in Israel and another 2,500 injured.

The Palestinian Health Ministry said Monday that at least 560 people have been killed and 2,900 others injured in retaliatory Israeli airstrikes on the Gaza Strip since Saturday.