Gaza militants have fired hundreds of rockets into southern Israel, killing at least four Israelis and bringing life to a standstill across the region in the bloodiest fighting since 2014.
As Israel pounded Gaza with airstrikes, the Palestinian death toll rose to 22, including two pregnant women and two babies.
The bloodshed marked the first Israeli fatalities from rocket fire since the 2014 war. With Palestinian militants threatening to send rockets deeper into Israel and Israeli reinforcements massing near the Gaza frontier, the fighting showed no signs of slowing down.
Israeli Prime Minister Benjamin Netanyahu spent most of the day huddled with his Security Cabinet.
Late on Sunday, the Cabinet instructed the army to "continue its attacks and to stand by" for further orders.
Israel also claimed to have killed a Hamas commander involved in transferring Iranian funds to the group.
Israel and Hamas, an Islamic militant group that seeks Israel's destruction, have fought three wars since Hamas violently seized control of Gaza from Western-backed Palestinian forces in 2007. They have fought numerous smaller battles, most recently two rounds in March.
While lulls in fighting used to last for months or even years, these flare-ups have grown increasingly frequent as a desperate Hamas, weakened by a crippling Egyptian-Israeli blockade imposed 12 years ago, seeks to put pressure on Israel to ease the closure.
The blockade has ravaged Gaza's economy, and a year of Hamas-led protests along the Israeli frontier has yielded no tangible benefits. In March, Hamas faced several days of street protests over the dire conditions.
With little to lose, Hamas appears to be trying to step up pressure on Mr Netanyahu at a time when the Israeli leader is vulnerable on several fronts.
Fresh off an election victory, Mr Netanyahu is now engaged in negotiations with his hard-line political partners on forming a governing coalition.
If fighting drags on, the normally cautious leader could be weakened in his negotiations as his partners push for a tougher response.
Later this week, Israel marks Memorial Day, one of the most solemn days of the year, and its festive Independence Day.
Next week, Israel is to host the Eurovision song contest. Prolonged fighting could overshadow these important occasions and deter foreign tourists.
The arrival of the Muslim holy month of Ramadan, which begins Monday, does not seem to be deterring Hamas.
But the group is also taking a big risk if it pushes too hard. During the 50-day war in 2014, Israel killed over 2,200 Palestinians, over half of them civilians, according to UN tallies, and caused widespread damage to homes and infrastructure.
While Hamas is eager to enhance its credentials as a resistance group, the Gazan public has little stomach for another devastating war.
"Hamas is the change seeker," said retired Brig Gen. Assaf Orion, a former head of the Israeli military general staff's strategic division. "Hamas needs to make its calculus, balancing its hope for improvement against its fear of escalation."
Tánaiste Simon Coveney "the indiscriminate rocket fire from Gaza".
Tánaiste @simoncoveney has issued a statement on the situation in the Middle East; calling for an end to indiscriminate attacks on civilians & for the international community to work with all sides to de-escalate tension as a matter of urgency.— Irish Foreign Ministry (@dfatirl) May 5, 2019
He called for an end to the attacks on civilians and said the "actions undermine the cause of the Palestinian people. The unjust treatment of Gaza and the suffering of people there will never be solved by violence or terrorist attacks."
In Washington, US Secretary of State Mike Pompeo said Israelis have "every right to defend themselves".
He expressed hope that the recent cease-fire could be restored.
The UN Middle East envoy, Nickolay Mladenov, called for a halt in rocket fire and "a return to the understandings of the past few months before it is too late".
EU foreign policy chief Federica Mogherini also called for a halt to "indiscriminate rocket attacks" from Gaza and expressed support for Egyptian and UN mediation efforts.
- Press Association