Palestinian militants bombarded southern Israel with dozens of rockets and mortar shells Tuesday, while Israeli warplanes struck targets throughout the Gaza Strip in the largest flare up of violence between the sides since a 2014 war.
The Israeli military said most of the projectiles were intercepted, but three soldiers were wounded, raising the chances of further Israeli retaliation. One mortar shell landed near a kindergarten shortly before it opened.
The sudden burst of violence follows weeks of mass Palestinian protests along the Gaza border with Israel. Over 110 Palestinians, many of them unarmed protesters, have been killed by Israeli fire in that time. Israel says it holds Gaza's Hamas rulers responsible for the bloodshed.
"Israel will exact a heavy price from those who seek to harm it, and we see Hamas as responsible for preventing such attacks," Israeli Prime Minister Benjamin Netanyahu said.
Israel and Hamas are bitter enemies and have fought three wars since the Islamic group seized control of Gaza in 2007.
The last war in 2014 was especially devastating, with more than 2,000 Palestinians killed, including hundreds of civilians, and widespread damage inflicted on Gaza's infrastructure in 50 days of fighting. Seventy-two people were killed on the Israeli side.
Tuesday's violence bore a striking resemblance to the run-up to past wars. In the early morning, Palestinian militants fired more than two dozen mortar rounds into southern Israel, including the shell that landed near the kindergarten.
The Israeli military said it carried out more than 35 airstrikes on seven sites across Gaza, including an unfinished tunnel near the southern city of Rafah that crossed under the border into Egypt and from there into Israeli territory. No Palestinian casualties were reported.
Palestinian militants continued to fire additional barrages toward southern Israel, setting off air raid sirens in the area throughout the day and into the evening.
Brig. Gen. Ronen Manelis, the chief military spokesman, threatened tougher action and said it was up to Hamas to stop the situation from escalating.
"These strikes will continue to intensify as long as necessary if this fire continues," he told reporters outside Israeli military headquarters.
Hamas and the smaller Islamic Jihad militant issued a joint statement Tuesday, claiming shared responsibility for firing rockets and projectiles against Israeli communities near Gaza.
They said Israel "began this round of escalation" by targeting their installations in the past two days, killing four militants. It was the first time the armed wing of Hamas has claimed responsibility for rocket attacks out of Gaza since the 2014 war.
Hamas has been severely weakened by the three wars with Israel, as well as a stifling Israeli-Egyptian blockade that has brought the local economy to a standstill.
Hamas initially billed the weekly border protests as a call to break through the fence and return to homes that were lost 70 years ago during the war surrounding Israel's establishment.
But the protests appear to be fueled primarily by a desire to ease the blockade. Gaza's unemployment rate is edging toward 50 percent, and the territory suffers from chronic power outages.
With limited options at its disposal, and a failure so far of the protests to significantly ease the blockade, Hamas appears to be gambling that limited rocket fire might somehow shake up the situation.
Ismail Radwan, a Hamas official, said the "resistance is capable of hurting the occupation and it proved this today by responding to its crimes."
Israel says the blockade is needed to prevent Hamas from building up its military capabilities.
Also Tuesday, two fishing boats carrying students and medical patients set sail from Gaza City's port, aiming to reach Cyprus and break the Israeli blockade, which has restricted most activity along the coast. Hamas acknowledged it was mostly a symbolic act.
One of the boats quickly turned around, while the Israeli navy intercepted the second vessel after it ventured beyond a six-mile (10-kilometer) limit imposed by Israel.
The Israeli military said the boat was intercepted without incident and was taken to the Israeli port of Ashdod, where the 17 people aboard would be sent back to Gaza.
In southern Israel, angry residents complained about the renewed rocket fire.
Adva Klein of Kibbutz Kfar Aza said she only got about two hours of sleep because of the frequent incoming fire and the warning sirens. Other residents reported machine- gun fire from Gaza.
"It's been a really scary morning," said Adele Raemer of Kibbutz Nirim.
Regional councils near the Gaza border instructed residents to stay close to bomb shelters.
The high Palestinian death toll in the border protests has drawn strong international criticism of Israel, with rights groups saying Israel's use of live fire is illegal because in many cases it has struck unarmed protesters who did not pose an imminent threat to Israeli soldiers.
But on Tuesday, the Palestinians came under criticism.
The United States condemned the attacks out of Gaza and called for an urgent meeting of the U.N. Security Council. U.S. Ambassador Nikki Haley said the Security Council "should be outraged and respond."
The European Union's foreign policy chief, Federica Mogherini, called for an immediate halt to the rocket and mortar fire.
"Indiscriminate attacks against civilians are completely unacceptable under any circumstances," she said.
The Israeli Foreign Ministry said it had instructed embassies around the world to seek similar condemnations of the Palestinian fire.
Israel has rejected the criticism of its response to the protests, saying it is defending its border and nearby communities. It accuses Hamas of trying to carry out attacks under the cover of protests and using civilian demonstrators as human shields.
Hamas has vowed to continue the border rallies.