Just past midnight on Monday, militants launched rockets at the southern Israeli cities of Beersheba and Ashkelon. Witnesses said Israeli planes launched dozens of air strikes. The Israeli military said a Hamas intelligence complex was among the targets.
Reuters footage captured several flashes illuminating buildings in the coastal enclave as the sound of explosions boomed through the city.
The Gaza Health ministry put the death toll in the densely populated enclave of two million Palestinians at 197, including 58 children and 34 women, over the last week. Ten people have been killed in Israel, including two children, Israeli authorities say.
On Sunday, diplomatic efforts to broker a cease-fire between Israel and Hamas ramped up as the death toll from the escalating conflict climbed.
After at least 37 Palestinians were killed Sunday by pre-dawn Israeli airstrikes on the Gaza Strip, Hamas said it was unleashing large missile barrages into Israel. Sirens warning of rockets wailed in the Israeli cities of Ashkelon and Ashdod, between Gaza and Tel Aviv. The Magen David Adom emergency service reported "direct hits."
The United Nations Security Council convened Sunday to discuss the worst outbreak of Israeli-Palestinian fighting in years. In opening remarks, U.N. Secretary-General António Guterres said the violence was "appalling" and had wrought "unconscionable death" and immense suffering.
"Fighting must stop. It must stop immediately," he said.
Despite the appeals, Israeli Prime Minister Benjamin Netanyahu said his military operation was continuing "at full force" despite any "international pressures."
"There are always pressures, but overall we get very serious backing," he said Sunday in an address on Israeli television, citing support from the U.S.
His comments came just hours after U.S. Department of State envoy Hady Amr, who was dispatched to the region for de-escalation talks with senior Israeli and Palestinian officials, met with Israeli Defense Minister Benny Gantz. Gantz also expressed gratitude for U.S. support.
In downtown Gaza City, ambulance teams and rescuers combed through the rubble left from pre-dawn airstrikes. Rescue teams were still searching for bodies and survivors into the early evening. Gaza's Health Ministry said the death toll of 37 could climb and that 50 people were injured.
The ministry's Dr. Yousef Abu Al-Rish warned that Gaza City's health system was nearing the point of collapse, telling NBC News that supplies were running low and damage to the streets around the main Al-Shifa hospital was impeding access for the injured.
The Israeli Defense Forces said in a statement on Sunday that the casualties were "unintended" after they struck an underground military facility, causing civilian houses above to collapse.
"The IDF aspires to avoid civilian casualties as far as possible but Hamas bears responsibilities for intentionally locating its military infrastructure under civilian houses, thus exposing civilians to danger," the statement said.
The medical charity Doctors Without Borders said one of its clinics also was damaged in the overnight bombardment. The organization said in a tweet on Sunday many of its staff and patients were forced to flee as "Israel continues its aerial and artillery bombardment."
"The damage to our clinic shows no place in Gaza is safe," Doctors Without Borders said.
In his remarks to the U.N. on Sunday, Guterres condemned the "immense suffering and damage to vital infrastructure."
"I am appalled by the increasingly large numbers of Palestinian civilian casualties, including many women and children, from Israeli strikes in Gaza," he said. "I also deplore Israeli fatalities from rockets launched from Gaza."
Since May 10, Hamas has fired more than 2,900 missiles at Israel, according to Israel's military. One man was killed in when a rocket exploded in the Tel Aviv suburb of Ramat Gan on Saturday.
The Israeli military also defended its decision to flatten the Al-Jala tower, which housed the offices of international media organizations including The Associated Press and Al Jazeera, saying the building was "an important base of operations" for Hamas military intelligence.
Press freedom organizations and the AP have called for an investigation and justification for the airstrike.
As casualties continued to climb, international leaders have expressed mounting alarm over the intensifying conflict and urged de-escalation. Saudi Arabia convened an emergency meeting of foreign ministers of the Organization of Islamic Cooperation for Sunday.
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The European Union's foreign policy chief also announced urgent talks with his bloc's foreign ministers to address the ongoing crisis and "unacceptable number" of civilian casualties.
"We will coordinate and discuss how the EU can best contribute to end the current violence,” Josep Borrell said in a statement on Twitter.
Pope Francis used his weekly address to appeal for calm, urging Israeli and Palestinian leaders to “stop the clamor of arms and walk the path of peace.”
Netanyahu, though, said the operation was "not over yet" — and will continue for "as long as it takes."