MT Desk: Palestinian Islamist group Hamas launched the biggest attack on Israel in years on Saturday, killing more than 20 people and wounding hundreds in a surprise assault that combined gunmen crossing into Israel with a barrage of rockets fired from Gaza.

Israel said the Iran-backed group had declared war as its army confirmed fighting with militants in several Israeli towns and military bases near Gaza and Prime Minister Benjamin Netanyahu vowed to retaliate.

“Our enemy will pay a price the type of which it has never known,” he said. “We are in a war and we will win it.”

At least 22 Israelis were killed in the attack so far, Israel’s ambulance service said. Around 545 Israelis had been wounded, the health ministry said.

The Israeli military said it had responded with air strikes into Gaza, where witnesses reported hearing heavy explosions and multiple dead being carried into hospitals.

The attack marked an unprecedented infiltration into Israel by an unknown number of Hamas gunmen crossing from the Gaza Strip, and the heaviest blow for Israel in the conflict with Palestinians since the suicide bombings of the Second Intifada some two decades ago.

Speaking to Israel N12 News by phone from Nir Oz, a kibbutz near Gaza, a woman identified as Dorin said militants had infiltrated her house and tried to open the bomb shelter where she was hiding.

“They just came in again, please send help,” she said. “There are a lot of homes harmed ... My husband is holding the door closed ... They are firing rounds of bullets.”

Israeli Defense Minister Gallant said “troops are fighting against the enemy at every location” and authorized the call-up of reservists.

Israeli media reported that gunmen had opened fire on passers-by in Sderot, and footage circulating on social media appeared to show clashes in city streets as well as gunmen in jeeps roaming the countryside.

“We were told there are terrorists inside the kibbutz, we can hear gunfire,” a young woman named Dvir, from Beeri Kibbutz, told Israeli Army Radio from her bomb shelter.

The escalation comes against a backdrop of surging violence between Israel and Palestinian militants in the West Bank, which together with the Gaza Strip is part of the territories where Palestinians have long sought to establish a state. Hamas media displayed videos of what it said were bodies of Israeli soldiers brought into Gaza by fighters, and Palestinian gunmen inside Israeli homes and touring an Israeli town in jeeps reportedly been driven into Israel by the attackers.

Reuters was not immediately able to verify the footage.

Hamas media also circulated video footage apparently showing a destroyed Israeli tank.

In Gaza, the roar of rocket launches could be heard and residents reported armed clashes along the separation fence with Israel, near the southern town of Khan Younis, and said they had seen significant movement of armed fighters.

Palestinians in Gaza were bracing for Israel’s response.

“We are afraid,” Palestinian woman, Amal Abu Daqqa, said as she left her house in Khan Younis.

Others in Gaza expressed disbelief at the infiltration into Israel. “It is like a dream. I still can’t believe it,” said one Gaza shopkeeper.

The attack came a day after Israel marked the 50th anniversary of the 1973 war that brought the country to the verge of catastrophic defeat in a surprise attack by Syria and Egypt.

Meanwhile, Israeli Prime Minister Benjamin Netanyahu has told Israel that it is “at war” with Hamas militants that rule the Gaza Strip.

Netanyahu’s comments in a televised address mark his first since the Gaza Strip’s Hamas rulers launched a major, multi-front attack on Israel at daybreak Saturday. He ordered a call-up of reservists and promised that Hamas would “pay a price that it hasn’t known until now.”

“We are at war,” Netanyahu said. “Not an ‘operation,’ not a ‘round,’ but at war.”

The prime minister also ordered the military to clear the infiltrated towns of Hamas militants that remained locked in gunfights with Israeli soldiers.

Hamas on Saturday fired thousands of rockets at Israel and sent dozens of fighters across the country’s heavily fortified border, a massive show of force that caught Israel off-guard on a major holiday.

Political commentators lambasted the government over its failure to anticipate what appeared to be a Hamas attack unseen in its level of planning and coordination.

The Israeli rescue service said that its medics were tending to 16 casualties in southern Israel, including a woman in her 60s who was killed when a rocket fired from Gaza made a direct hit, and two people in serious condition.

There were reports of many more casualties on both sides, but authorities did not immediately release details. Israeli media reported that dozens of people were hospitalized in southern Israel. The Palestinian Health Ministry in Gaza reported injuries among “many citizens” without giving numbers and loudspeakers on mosques broadcast prayers of mourning for killed militants.