A 13-story residential block collapsed in Gaza on Tuesday following an Israeli air strike, as the death toll in the spiralling crisis rose to 31.

Hostilities between Israel and the Palestinian Islamist group that runs Gaza escalated with each side launching a series of aerial bombardments in some of the region’s worst violence since the last major conflict in 2014.

Footage showed a barrage of rockets flying through skies over Israel, as world leaders’ calls for calm were ignored.

People on the ground said residents in the block had been warned to evacuate around an hour before the huge explosion, amid dozens of air strikes.

Israel reported explosions and sirens more than 70 km (45 miles) up the coast from Gaza.

28 people have so far been reported dead in Gaza and three in Israel and there have been hundreds injured during days of violence.

Late into the night, Gazans reported their homes shaking and the sky lighting up with near-constant Israeli strikes.

It follows days of unrest after clashes between Palestinians and Israeli police at Jerusalem’s al-Aqsa Mosque during the holy month of Ramadan.

It comes after hundreds of Palestinians were injured on Monday.

Among a number of children killed yesterday were young brothers Ibrahim and Marwan, who were filling sacks of straw in northern Gaza when an Israeli strike came from the skies, killing them and three other relatives.

Bloodstains could still be seen on Tuesday outside their house in Beit Hanun, where their cousin, also called Ibrahim, witnesses the killing.

130 missiles targeted at Tel-Aviv

He and the two brothers, aged 10 and 14, ‘had prepared three sacks, and I had just left to take one to put on the cart,’ he told AFP.

‘We were laughing and having fun, when suddenly they began to bomb us, everything around us caught fire.’

‘I saw my cousins set alight, and torn into pieces,’ the 14-year-old explained, breaking down in tears.

Their cousins, three-month old baby Yazan and 10-year-old girl Rahaf, were also killed in the strike.

Another family member, 22-year-old Ahmad, died later from his injuries. He had been due to get married on Saturday.

Sitting in a cemetery at a funeral for the victims was Abu Hussein Hamad, stroking the face of his 11-year-old son, Hussein, for the last time before he was buried.

‘What did this innocent child do?’ he asked, crying.

Israel reported that 480 rockets had been fired across the border by Palestinian militant groups, sending entire Israeli communities running to air raid shelters, even before the barrage of rockets fired in retaliation for the tower block bombing.

Gazans walk amid rubble of tower that collapsed in Israeli air strike

The White House condemned the rocket attacks, and said Israel had a legitimate right to defend itself, but that the primary U.S. focus was on de-escalation.

The International Committee of the Red Cross urged all sides to step back, and pointed to the requirement in international law to avoid civilian casualties where possible.

But Israeli Prime Minister Benjamin Netanyahu said he would step up the strength and frequency of strikes on Gaza, an enclave of 2 million people, in response to the rocket attacks.

Israel says it had sent 80 jets to bomb Gaza, and dispatched infantry and armour to reinforce the tanks already on the border.

That has sparked comparisons with the last Israeli incursion into Gaza to stop rocket attacks, in 2014.

More than 2,100 Gazans were killed in the seven-week war that followed, according to the Gaza health ministry, along with 73 Israelis, while thousands of homes in Gaza were razed.

Video footage on Tuesday showed three plumes of thick, black smoke rising from the Gaza block as it toppled over. Electricity in the surrounding area also went out.

There were no reports of casualties two hours after it collapsed.

Shortly after the attack, Hamas and the Islamic Jihad group said they would respond by firing rockets at Tel Aviv.

Air raid sirens and explosions were heard around the city, and the skies were lit up by the streaks of multiple interceptor missiles launched towards the incoming rockets.

Pedestrians ran for shelter, and diners streamed out of Tel Aviv restaurants while others flattened themselves on pavements as the sirens sounded.

The Israel Airports Authority said it had halted take-offs at Tel Aviv airport ‘to allow defence of the nation’s skies’. Video broadcast on Israeli television showed interceptor missiles rising above the runways.

Israel’s Magen David Adom ambulance service said a 50-year-old woman was killed when a rocket hit a building in the Tel Aviv suburb of Rishon Lezion, and that two women had been killed in rocket strikes on the southern city of Ashkelon.

Meanwhile, there were clashes between pro-Palestine and pro-Israel protesters outside Downing Street in London on Tuesday.

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MORE : ’20 killed including children in Israel air strikes’ after Hamas fires rockets