Israeli forces waged ground operations against Hamas in Gaza on Sunday in what Israeli Prime Minister Benjamin Netanyahu called the second phase of a three-week-old war aimed at crushing the Palestinian militant group.

Gaza's besieged residents faced a near-total communications and Internet blackout as Israel's warplanes dropped bombs and its troops and armour pushed into the Hamas-ruled enclave, with Israeli military chiefs signalling they were gearing up for an expanded ground offensive.

Speaking at a press conference in Tel Aviv on Saturday, Netanyahu warned Israelis to expect a "long and hard" campaign but stopped short of calling the current incursions an invasion.

Some of US President Joe Biden's aides have advised Israeli counterparts to hold off on an immediate all-out assault, US officials have said.

Even as initial ground operations appeared limited for now, Netanyahu pledged to spare no effort to free the more than 200 hostages, including Americans and other foreigners, held by Hamas.

"This is the second stage of the war whose goals are clear - to destroy Hamas' governing and military capabilities and to bring the hostages home," Netanyahu told reporters.

"We are only at the start," he said. "We will destroy the enemy above ground and below ground."

Israel has tightened its blockade and bombarded Gaza for three weeks since the Islamist group Hamas' devastating Oct. 7 attack. At least 1,400 Israelis were killed in the deadliest day of the nation's 75-year history, Israeli authorities said.

Western countries have generally backed what they say is Israel's right to self-defence. But there has been a mounting international outcry over the toll from the bombing and growing calls for a "humanitarian pause" to allow aid to reach Gaza civilians and ease the humanitarian crisis.

Medical authorities in the Gaza Strip, which has a population of 2.3 million people, say 7,650 Palestinians have been killed in Israel's campaign to obliterate the Iran-backed militants.

President Mahmoud Abbas, whose Palestinian Authority governs parts of the occupied West Bank while Hamas rules Gaza, said, “Our people in the Gaza Strip are facing a war of genocide and massacres committed by the Israeli occupation forces in full view of the entire world.” With many buildings reduced to rubble and shelter hard to find, Gazans are short of food, water, fuel and medicines. Their plight got worse from Friday night when phone and internet services were cut - followed by heavy bombing through the night.

The communications outage persisted into Sunday.

"God help anyone under the rubble," said one Gaza journalist, who spent a terrifying night in a building stairway as bombs fell and Israeli forces appeared to exchange fire with Palestinian fighters.

Israel's chief military spokesperson declined to say whether Israel was behind the telecommunications blackout in Gaza but said it would do what it needed to protect its forces.

Targeting maze of Hamas tunnels

Israel sent troops and tanks into Gaza on Friday night, focusing on infrastructure including the extensive tunnel network built by Hamas, the Israeli military said. It provided no details on the size of the deployment.

Netanyahu on Saturday reiterated Israel's call for Palestinian civilians to evacuate the northern Gaza Strip where Israel was focusing its attack on what it was were Hamas hideouts and other installations.

But Palestinians say nowhere is safe, with bombs also smashing homes in the south of the densely populated territory.

"A humanitarian catastrophe is unfolding in front of our eyes," UN Secretary-General Antonio Guterres said. The UN Security Council plans to meet on Monday on the Israel-Gaza crisis, diplomats said.

Billionaire entrepreneur Elon Musk offered his SpaceX's Starlink satellite network to support communications in Gaza for "internationally recognized aid organisations." Israel responded that it would fight the move, saying Hamas would "use it for terrorist activities."

Netanyahu, who met with hostages' families on Saturday, said contacts to secure their release would continue even during a ground offensive and that military pressure on Hamas could help bring them home. He did not elaborate.

Qatar-mediated negotiations between Israel and Hamas continued but at a much slower pace than before Friday's escalation in Gaza, a source briefed on the talks said.

Hamas' armed wing said its fighters battled Israeli troops in northeastern and central Gaza. "Al-Qassam Brigades and all Palestinian resistance forces are fully prepared to confront the aggression with full force and thwart the incursions," it said.

The United States and other Western countries had urged Israel to hold off on launching a major ground offensive for fear of high Palestinian casualties and a widening conflict, as well as to give more time for hostage negotiations. But US officials said it was up to Israel to make its own decisions.

Amid fears that the Israel-Hamas war could spiral into a regional conflict, Defence Minister Yoav Gallant told reporters Israel had no interest in expanding the fighting beyond Gaza but is prepared on all fronts.

The crisis brought hundreds of thousands of pro-Palestinian demonstrators out in cities around Europe, the Middle East and Asia on Saturday. (Reuters-Yonhap)