With communities across the Gaza Strip already laid to waste by a relentless barrage of airstrikes in retaliation for the Hamas surprise attack on Israel, conditions for Palestinians grew even more dire on Wednesday when the only power plant ran out of fuel, leaving the territory with no electricity and running water, officials said.

The shutdown of the Gaza Electricity Power Plant "creates a humanitarian crisis" for the 2.3 million residents packed in the 140 square miles comprising the Gaza Strip, Gaza authorities said in a statement Wednesday.

"This threatens to plunge the Strip into complete darkness and make it impossible to continue providing all basic life services, all of which depend on electricity, and it will not be possible to operate them partially with generators in light of the prevention of fuel supplies from Rafah Gate," according to the statement, which included, an "urgent appeal to the international community and its humanitarian and relief organizations to act quickly to stop this crime against humanity and this mass killing that has taken on many forms."

"We stress the need to provide the Gaza Strip with all means of life," the authorities said.

But Lt. Col. Jonathan Conricus, spokesman for the Israeli Defense Forces, told ABC News Wednesday night that there is "no room for mercy" for Hamas. Conricus said it is not the intention of the Israeli forces to harm civilians in Gaza, but added that Hamas has been using civilians as "human shields" and hiding in residential areas.

He said Israel is determined to exact revenge for the atrocities Hamas committed when it stormed the border into Israel on Saturday morning and indiscriminately killed Israeli civilians.

As of Wednesday night, at least 1,200 people have died and 2,900 others have been injured in Israel since the horrific incursion commenced. Israeli officials said the armed militants went from town to town and kibbutz to kibbutz slaughtering men, women and even babies.

At a desert music festival near the Gaza border, Hamas gunmen killed 260 Israeli young people and abducted others, taking them back into Gaza, Israeli officials said. In other border communities, the terrorists went door to door, gunning down Israeli civilians and taking women, the elderly and children captive, officials said.

"If Hamas is able to get away with murdering 900 Israelis, to see the light of another day, that isn't the message that we can afford and allow ourselves to tell the other extremist organizations," Conricus told ABC News.

The power blackout in Gaza came a day after the Israeli government ordered a "total siege" of the neighboring Gaza Strip, allowing no food, fuel or electricity to enter the Hamas-ruled territory.

The United Nations said the number of Palestinians displaced from their homes by the shelling is at least 263,000. The evacuees were seeking shelter in a U.N. school and at a hospital.

United Nations Relief and Works Agency in Gaza said it has not been immune to the bombings. The UNRWA said 18 of its facilities in Gaza, including schools sheltering displaced civilians, have been damaged by the Israeli retaliatory airstrikes.

The Palestine Red Crescent Society, a humanitarian organization in Gaza, said four of its paramedics were killed in the Israeli retaliatory airstrikes.

"PRCS demands accountability for this war crime, urging immediate investigation and justice for the victims," the agency said in a statement. "Targeting medical personnel is a grave breach to international humanitarian law and to humanity."

The airstrikes were launched by Israel following the surprise and highly coordinated attacks by Hamas militants on Saturday morning. Since then, hundreds of targets have been hit in Gaza.

More than 200 targets were struck in Gaza by Israeli forces in just one day, according to Israeli officials.

At least 950 people have died and 5,000 others have been injured in Gaza since the airstrikes by Israel began, the Palestinian Health Ministry said. At least 260 of the dead in Gaza are children and 230 are women, the health ministry said.

As of Wednesday night, more than 1,200 civilians and soldiers had been killed in Israel and more than 2,900 injured since Saturday's surprise attack by Hamas, Israeli officials said.

Israeli officials said Hamas showed no mercy to Israeli civilian families at the Kibbutz Kfar Aza in southern Israel, where fathers, mothers and their children were found slain in their homes.

Inside some of the houses, soldiers found bodies of entire families, Israeli Maj. Gen. Itai Veruv told ABC News during a ghastly tour of the shattered community on Tuesday.

"You see the babies, the mothers, the fathers in the bedrooms, in the protection rooms and how the terrorists killed them," Veruv said. "It's not a war, it's not a battlefield. It's a massacre."

Describing the gruesome discoveries soldiers made in the houses, Veruv said, "They burned the apartments, then they shoot the babies, they cut their heads."

In Gaza, Doctors Without Borders, an independent medical humanitarian group, said that all of the patients treated at its clinic in Gaza City on Wednesday were children ages 10 to 14.

"This is because the majority of the injured in Gaza are women and children since they are the ones who are most often in the houses that get destroyed in the airstrikes," Doctors Without Borders said in a statement.

In the hours after his country was attacked, Israeli Prime Minister Benjamin Netanyahu declared war on Hamas.

On Tuesday, Israeli Defense Minister Yoav Gallant toured southern Israel along the Gaza border, telling soldiers they are moving to "a full-scale response" to the Hamas surprise attack and that he has "removed every restriction."

"Hamas wanted to see a change in Gaza -- the reality is Gaza will make a 180. They will regret [their actions]," Gallant said.

President Joe Biden has said repeatedly since the Hamas attack that the United States fully supports Israel's right to defend itself and will offer its assistance in that endeavor.

But with concerns mounting over civilian casualties in Israel and Gaza, Biden, speaking to a gathering of Jewish community leaders on Tuesday, recounted what he said was a conversation this week with Netanyahu.

"One thing that I did share is that it is really important that Israel with all the anger, frustration ... that exists, is that they operate by the rules of war. And there are rules of war," Biden said.

U.S. National Security Council spokesman John Kirby said Wednesday that the United States is working with Israel and Egypt to ensure civilians in Gaza can get safe passage out.

"We believe that safe passage is important, and we want to see safe passage, a safe passage corridor opened up. We also believe it's important that humanitarian assistance have a way to continue to get to the Palestinian people," Kirby said.

Kirby stressed the importance of getting humanitarian assistance to civilians in Gaza, saying the U.S. is "going to continue to pursue options to make sure that they get that humanitarian assistance."

Kirby said U.S. officials have been in communication with the Palestinian Authority.