Thousands of Gazans have fled their homes in search of safety following the intensification of Israeli airstrikes in the besieged enclave and the first ground incursion.

At least 17,000 displaced people have come to the United Nation's body for Palestinian refugees, UNRWA, seeking shelter, marking a "dramatic rise," a spokesman said Sunday. He added that in the past 24 hours nine UNRWA installations, all schools, have been damaged in Gaza.

"We call on both sides to abide by their obligations under international humanitarian law. There must be respect for the sanctity of civilian life and the inviolability of U.N. installations," UNRWA spokesman Chris Gunness said. "The parties to the conflict know the exact location of all UNRWA installations in Gaza. The Israeli army has the precise GPS coordinates."

Israel's offensive in Gaza — its largest in the territory since 2012 — is now into its sixth day, despite growing international pressure for a ceasefire amid rising civilian casualties.

At least 172 Palestinians had been killed in airstrikes, Gaza's Health Ministry said Sunday, adding that at least 29 were children under the age of 16.

Gaza's spike in internally displaced people followed a warning by the Israeli military to residents in the territory's north, telling them to leave their homes ahead of an impending attack:

"Those who fail to comply with the instructions to leave immediately will endanger their lives and the lives of their families. Beware."

A senior military officer told reporters that Israel would "strike with might" in the Beit Lahiya area from the late evening hours on Sunday. He did not specify whether it would include a ground operation.

Meanwhile, rockets fired from Gaza have reached further into Israel than ever before, setting off sirens in major Israeli cities such as Tel Aviv and Haifa. No Israelis have been killed, though several injuries have been reported.

Israel Defense Forces (IDF) said 72 rockets had been fired from Gaza on Sunday, and 12 had been intercepted by Israel's Iron Dome system, which leaders decided to expand with additional batteries after the barrage, Israeli news website Haaretz reported. On Sunday, an Israeli teenager was wounded when a rocket hit the southern town of Ashkelon, according to emergency services, Haaretz added.

Four Israeli soldiers were wounded while taking part in the first ground attack of the offensive, a dawn raid on Sunday that the military said was meant to destroy a site being used to fire rockets.

"During the mission a gunfight broke out, started by some terrorists operating at the site, during which four Israeli soldiers were lightly wounded," the spokesman said.

The armed wing of Hamas, the Qassam Brigades, said there was an exchange of gunfire "between our fighters and soldiers of the Zionist navy which tried to penetrate the zone of Sudaniya," located in northwest Gaza. They added that Qassam fighters had prevented the Israeli soldiers from landing.

Israeli Prime Minister Benjamin Netanyahu said Sunday that the country would “continue to act with patience, forbearance, with determination, responsibility and aggression to achieve the goal of the campaign — restoring calm for a long period by dealing a significant blow to Hamas and other terrorist groups in the Gaza Strip."

Earlier Sunday, the IDF said an explosion in the occupied Golan Heights appeared to have been from a rocket fired from Syria, Haaretz said. In response, Israel said the military fired at targets in Syria. Missiles were also fired from southern Lebanon, leading to a return of fire from the IDF across the border.

Sunday’s exchange of rocket fire took place despite a plea the day before from the UN Security Council for a ceasefire.

International pressure has intensified amid increasing civilian deaths in Gaza, with world leaders calling on both sides to show restraint and respect international law.

United States Secretary of State John Kerry is expected to discuss prospects of a ceasefire between Israel and Hamas Sunday evening with his counterparts from France, Germany, and England in Vienna, where the ministers are currently attending nuclear talks with Iran. Netanyahu has so far publicly spurned the idea of ceasefire with Hamas, saying the IDF is ready for a long engagement.

Any diplomatic effort to bring both sides to a ceasefire will have to take place against a backdrop of increased violence.

On Saturday, an Israeli airstrike on the home of Gaza City policeman Tayseer Al-Batsh killed at least 18 members of the family in the single-deadliest attack since the offensive began. The IDF said it suspected the house was harboring operatives planning a rocket attack, adding it had launched an investigation into the incident, Israeli media reported. Israel also struck a center for the disabled early Saturday in Gaza, killing at least two.

The latest tensions between Israel and Hamas began soon after Israel backed out of peace talks and on news that rival Palestinian factions Hamas and Fatah agreed to form a unity government. Negotiations were already faltering due to Israel's reluctance to rein in plans to expand settlements in the occupied territories, considered illegal under international law.

The situation on the ground deteriorated when three Israeli teenagers disappeared in the West Bank, with Israel blaming Hamas for their abduction — an accusation Hamas has denied. In the weeks that followed, Israel launched a crackdown on the West Bank with the stated goals of bringing back the boys and punishing Hamas.

But reports surfaced last week that a gag order had prevented information about the kidnappings from being publicized, including evidence that the boys may have been shot and killed. By the time that their bodies were found, at least 6 Palestinians had been killed as a direct result of the ensuing Israeli raids across the West Bank, and hundreds arrested including many Hamas members.

After news of the teenagers’ deaths did become public on June 12, a Palestinian teenager was abducted and burned alive. It was suspected that Israeli ultra-nationalists were responsible, and three people have reportedly confessed to the crime.

Scattered cross border attacks by Hamas and Israel soon escalated, and after Israel amassed forces along the Gaza border it launched Operation Protective Edge on July 7.

With wire services