GAZA/JERUSALEM (Reuters) – Hostilities between Israel and the Palestinian Islamist group that runs Gaza escalated on Tuesday as each side attacked the other with aerial bombardments that recalled their last major conflict in 2014.

A 13-story residential block in Gaza collapsed after one of several dozen Israeli air strikes, albeit after an Israeli warning, as Israelis reported explosions and sirens more than 45 miles up the coast from Gaza. Thirty-one people were reported dead: 28 in Gaza and three in Israel.

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

The fighting between Israel and Gaza’s armed factions was triggered by clashes between Palestinians and Israeli police at Jerusalem’s al-Aqsa Mosque on Monday.

Even before a barrage sent in retaliation for the destruction of the tower block, which contained a civilian Hamas office, Israel reported that 480 rockets had been fired across the border by Palestinian militant groups, sending entire Israeli communities running to air raid shelters.

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 reminded them of the requirement in international law to try to avoid civilian casualties.

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 said it had sent 80 jets to bomb Gaza, and dispatched infantry and armor to reinforce the tanks already gathered on the border.

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.

“We are now carrying out our promise,” Hamas’s armed wing said. “The Qassam Brigades are launching their biggest rocket strike against Tel Aviv and its suburbs, with 130 rockets, in response to the enemy’s targeting of residential towers.”

The Israeli military said many of the rockets fired from Gaza had fallen short and wounded Palestinians, and that Israel’s Iron Dome air defenses had intercepted the bulk of those that made it across the border.

Jerusalem—holy to Jews, Muslims and Christians—has been tense during the Muslim fasting month of Ramadan, with the threat of a court ruling evicting Palestinians from homes claimed by Jewish settlers adding to the friction.

In the last few days that tension has exploded into violence at al-Aqsa Mosque, which stands in what Muslims call the Noble Sanctuary, and Jews the Temple Mount.