Militants in Gaza and Israeli forces engaged in a bloody and spiralling clash over the weekend, with Palestinian factions launching hundreds of rockets towards towns and cities in Israel, which retaliated with more than 250 strikes.

In exchanges that marked some of the worst fighting in recent years, 19 Palestinians, including two pregnant women and a toddler, have been killed since hostilities began on Friday, the health ministry in Gaza said. The dead included at least eight militants and a Hamas commander killed in the first targeted assassination Israel has conducted in the strip for years.

Four Israelis were reported as killed by rockets, the highest death toll on the Israeli side since the 50-day war in 2014.

On Sunday night, Donald Trump tweeted his “100%” support for Israel’s actions after “a barrage of deadly rocket attacks by terrorist groups Hamas and Islamic Jihad”.

He added: “To the Gazan people — these terrorist acts against Israel will bring you nothing but more misery. End the violence and work towards peace – it can happen!”

The violence is following a pattern established over the past year in which short-lived exchanges have erupted on a near-monthly basis, usually dying down quickly under Egyptian and UN mediation. Late on Sunday a TV station belonging to Hamas claimed a ceasefire had been reached, however, the battling sides vowed to pursue each other aggressively on Sunday and moved further than in previous flare-ups.

Benjamin Netanyahu ordered the armed forces to conduct “massive strikes” on Gaza and reinforce the edge of the enclave with tanks, artillery and soldiers.

“This morning, I instructed the [military] to continue massive attacks against terrorist elements in the Gaza strip. I also ordered the reinforcement of the units around the Gaza strip with armour, artillery and infantry,” the prime minister said.

Facebook Twitter Pinterest A woman is loaded into an ambulance by Israeli emergency personnel after a rocket attack in the southern town of Ashdod. Photograph: Ahmad Gharabli/AFP/Getty Images

The Israel Defence Forces (IDF), which had halted its targeted assassination to avoid escalating tensions, said it killed Hamas commander Ahmed Khudari in an operation, believed to be a strike on a car. The army said Khudari was responsible for cash transfers from Iran to militants in Gaza.

The IDF said it had also targeted Palestinian militants inside a weapons warehouse. In another attack, an IDF attack helicopter shot at a militant, and the army said it was bombing “weapons warehouses hidden in houses of terror operatives”.

In previous rounds the Israeli air force has sought to avoid high casualties, even among militants, often telling residents of Gaza by phone that they are going to bomb a building so it can be emptied.

Hamas, which rules the strip, and a separate armed group, Palestinian Islamic Jihad, continued to fire off rockets late into Sunday evening. Israel said more than 600 had been launched in total and the militant factions put out a statement threatening to use longer-range rockets to bombard deeper into Israel. “The resistance decided to respond to the crimes of the occupation in an unprecedented manner,” it said.

Israel’s anti-missile Iron Dome system had intercepted many of the projectiles but several landed in residential neighbourhoods. In one strike, a primary school was hit and a hospital was damaged by shards of debris from an intercepted rocket. Several Israelis, including an 80-year-old woman, were wounded.

In Gaza, the dead included a pregnant woman and her 14-month-old niece. Israel denied responsibility for their deaths, saying they were hit by accidental militant fire. The health ministry in Gaza said on Sunday evening a second woman, who was nine months pregnant, had been killed in an Israeli airstrike. The IDF said it would not comment.

Israel’s bombing campaign also flattened a building containing the Gaza office of the Turkish state-run news agency, an attack the Turkish president, Recep Tayyip Erdoğan, said was an act of “terrorism”. The IDF said it had hit a building in Gaza City containing Hamas military intelligence and general security offices.

Fatima Al-Zaeem, a 45-year-old from Gaza City, said she and her four children had been hiding from the strikes all weekend. “We are not heroes, we are the victims of aggression and siege,” she said. “It’s a situation of horror for two days now. I’m a mother of four kids and we are on the eve of Ramadan.”

Fighting is often sparked by an incident that ignites tensions. On Friday, Palestinian militants fired across the border, wounding two soldiers, which Israel responded to with airstrikes, killing two Hamas militants. Israeli snipers also killed two people in Gaza demonstrating near the frontier.

UN peace envoy Nickolay Mladenov has since called for calm.

“Continuing down the current path of escalation will quickly undo what has been achieved and destroy the chances for long-term solutions to the crisis,” he said in a statement. “This endless cycle of violence must end, and efforts must accelerate to realise a political solution to the crisis in Gaza.”

Jeremy Stoner, Middle East regional director for the aid agency Save the Children, said the humanitarian community “believes we may have entered the most serious stage in this crisis since the 2014 Gaza war.

“We echo the UN special coordinator for the Middle East peace process’s call on all parties to immediately de-escalate the situation,” he added.

An Egyptian-led ceasefire agreement was designed to ease the conditions of a 12-year air, land and sea blockade on Gaza while halting rocket fire into Israel. Following the rocket strikes, Israel closed entry and exits to Gaza and restricted its fishing zone.

The escalation comes as Israel prepares to host the high-profile Eurovision song contest in Tel Aviv. The week of the competition will also mark the one-year anniversary since Israeli snipers killed nearly 60 Palestinians protesting at the frontier.

The 140 sq mile strip is home to about 2 million people but has been blockaded by Israel and Egypt since Hamas took control in 2007. Israel and Hamas have fought three wars and the economy has imploded, with youth unemployment over 70%. Hamas’s rival, the Palestinian Authority, based in the occupied West Bank, has also imposed sanctions on the strip, cutting off money transfers and deepening the crisis.