At least 35 people have been killed including 10 children in Gaza and three in Israel as hostilities in the Palestinian conflict erupted.
In the most intensive violence in years jets bombed Gaza and hundreds of rockets hit Israel.
Israel said it had sent 80 jets to bomb Gaza, and dispatched infantry and armour to reinforce the tanks already gathered on the border.
Video footage on Tuesday showed three plumes of thick, black smoke rising from a 13-storey Gaza residential and office block as it toppled over after being demolished by Israeli air strikes.
One multi-storey residential building in Gaza collapsed and another was heavily damaged after they were repeatedly hit by Israeli air strikes early on Wednesday.
The Israeli military said the building, in Gaza City's Rimal neighbourhood, housed "multiple" Hamas offices, including ones for military research, and development and military intelligence.
Residents in the block and the surrounding area had been warned to evacuate the area before the air strike, according to the military.
A second residential and office building in the same neighbourhood was heavily damaged in Israeli attacks shortly before 2 a.m. on Wednesday.
Gaza health ministry officials put the death toll at 32, but a Hamas-affiliated radio station later said three more people, including a woman and a child, were killed shortly before 2 a.m. on Wednesday in an Israeli air strike on an apartment above a restaurant.
Israeli political leaders and the military said they had killed "dozens" of militants, and hit buildings used by Hamas.
Defence Minister Benny Gantz said Israel had carried out "hundreds" of strikes, and that "buildings will continue to crumble."
Gaza's health ministry said that, of the reported dead, 10 were children and one was a woman.
Meanwhile, Palestinian militant groups fired multiple rocket barrages at Tel Aviv and Beersheba.
Hamas's armed wing said it fired 210 rockets towards Beersheba and Tel Aviv in response to the bombing of the buildings in Gaza City.
In Tel Aviv, air raid sirens were heard around the city.
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 Ashkelon.
It was some of the worst offensive strikes between Israel and Hamas since a 2014 war in Gaza, and sparked international concern that the situation could spiral out of control.
U.N. Middle East peace envoy Tor Wennesland said on social media: "Stop the fire immediately.
"We're escalating towards a full scale war. Leaders on all sides have to take the responsibility of de-escalation.
"The cost of war in Gaza is devastating & is being paid by ordinary people. UN is working w/ all sides to restore calm. Stop the violence now."
The violence followed weeks of tension in Jerusalem during the Muslim fasting month of Ramadan, with clashes between Israeli police and Palestinian protesters in and around Al-Aqsa Mosque, on the compound revered by Jews as Temple Mount and by Muslims as the Noble Sanctuary.
These escalated in recent days ahead of a - now postponed - court hearing in a case that could end with Palestinian families evicted from East Jerusalem homes claimed by Jewish settlers.
Israeli Prime Minister Benjamin Netanyahu earlier warned that militants would pay a "very heavy'' price for the rockets, which reached the outskirts of Jerusalem on Monday during a holiday in Israel commemorating its capture of East Jerusalem in a 1967 war.
The Arab League, some of whose members have warmed ties with Israel over the last year, accused it of "indiscriminate and irresponsible" attacks in Gaza and said it was responsible for "dangerous escalation" in Jerusalem.
Hamas group leader, Ismail Haniyeh, said Israel had "ignited fire in Jerusalem and Al-Aqsa and the flames extended to Gaza, therefore, it is responsible for the consequences."
Ismail said that Qatar, Egypt and the United Nations had been in contact urging calm but that Hamas's message to Israel was: "If they want to escalate, the resistance is ready, if they want to stop, the resistance is ready."
The US said on Tuesday that Israel had a legitimate right to defend itself from rocket attacks but applied pressure on Israel over the treatment of Palestinians, saying Jerusalem must be a place of coexistence.
State Department spokesman Ned Price urged calm and "restraint on both sides", saying: "The loss of life, the loss of Israeli life, the loss of Palestinian life, It's something that we deeply regret."
He added: "We are urging this message of de-escalation to see this loss of life come to an end."