Hamas released two elderly Israeli women held hostage in Gaza on Monday, as the United States expressed increasing concern that the escalating Israel-Hamas war will spark a wider conflict in the region, including attacks on American troops.

The death toll in Gaza was rising rapidly as Israel ramped up airstrikes, flattening residential buildings in what it says was preparation for an eventual ground assault.

The United States advised Israel to delay an expected ground invasion to allow time to negotiate the release of more hostages.

A third small aid convoy from Egypt entered Gaza, where the population of 2.3 million has been running out of food, water and medicine under Israel's two-week seal. With Israel still barring entry of fuel, the UN said its distribution of aid would grind to a halt within days when it can no longer fuel its trucks.

Gaza hospitals are struggling to keep generators running to power life-saving medical equipment and incubators for premature babies.

The release of the two hostages, 85-year-old Yocheved Lifshitz and 79-year-old Nurit Cooper, was confirmed by the International Committee of the Red Cross.

The two women, along with their husbands, were snatched from their homes in the kibbutz of Nir Oz near the Gaza border in Hamas' October 7 rampage through towns of southern Israel. Their husbands were not released.

In a statement, Hamas said it had released them for humanitarian reasons. Hamas and other militants in Gaza are believed to have taken roughly 220 people, including an unconfirmed number of foreigners and dual nationals. Hamas released an American woman and her teenage daughter last week.

Israel is widely expected to launch a ground offensive in Gaza, vowing to destroy Hamas after its brutal October 7 rampage into southern Israeli communities. That is raising fears of the war spreading beyond Gaza and Israel, as Iranian-backed fighters in the region are warning of possible escalation, including targeting US forces deployed in the Mideast.

The US has told Iranian-backed Hezbollah in Lebanon and other groups not to join the fight.

Israel has frequently traded fire with Hezbollah, and Israeli warplanes have struck targets in the occupied West Bank, Syria and Lebanon in recent days.

National Security Council spokesman John Kirby said there had been an uptick in rocket and drone attacks by Iranian-backed militias on US troops in Iraq and Syria, and the US was “deeply concerned about the possibility for any significant escalation” in attacks in coming days.

He said US officials were having “active conversations” with Israeli counterparts about the potential ramifications of escalated military action.

The US advised Israeli officials that delaying a ground offensive would give Washington more time to work with regional mediators on securing the release of more hostages taken by Hamas, according to a US official.

Israeli Defence Minister Yoav Gallant told troops near Gaza to keep preparing for an offensive “because it will come.” He said it will be a combined offensive from air, land and sea but did not give a timeframe.

Tanks and troops have been massed at the Gaza border, and Israel says it has stepped up airstrikes to reduce the risk to troops in the next stages. A ground excursion is likely to dramatically increase casualties in what is already the deadliest by far of five wars fought between Israel and Hamas since the militant seized power in Gaza in 2007.

More than 1,400 people in Israel have been killed — mostly civilians slain during the initial Hamas attack. At least 222 people were captured and dragged back to Gaza, including foreigners, the military said Monday, updating a previous figure.

More than 5,000 Palestinians, including some 2,000 minors and around 1,100 women, have been killed, the Hamas-run Health Ministry said Monday. That includes the disputed toll from an explosion at a hospital last week.

The toll has climbed rapidly in recent days, with the ministry reporting 436 additional deaths in just the last 24 hours.

Israel said it had struck 320 militant targets throughout Gaza over the last 24 hours. The military says it does not target civilians, and that Palestinian militants have fired over 7,000 rockets at Israel since the start of the war.

The Israeli military released footage showing what it said were attacks on Hamas infrastructure, with explosions as multi-story buildings collapsed or toppled over.

Israel carried out limited ground forays into Gaza. On Sunday, Hamas said it destroyed an Israeli tank and two armoured bulldozers inside Gaza. The Israeli military said a soldier was killed and three others were wounded by an anti-tank missile during a raid inside Gaza.

The military said the raid was part of efforts to rescue hostages. Hamas hopes to trade the captives for Palestinian prisoners held by Israel.

On Monday the Palestinian Red Crescent said 20 trucks entered Gaza carrying food, water, medicine and medical supplies, through the Rafah crossing with Egypt, the only way into Gaza not controlled by Israel. It was the third delivery in as many days, each around the same size.

The aid coming in so far is “a drop in the ocean” compared to the needs of the population, said Thomas White, the Gaza director of the UN agency for Palestinian refugees, UNRWA. The UN has said 20 trucks amounts to 4 per cent of an average day's imports before the war and that hundreds of trucks a day are needed.

White said the agency had only three days of fuel left for its trucks. The supplies coming through Rafah are reloaded onto UNRWA and the Red Crescent trucks to take to hospitals and UN schools in the south of Gaza, where hundreds of thousands of people are taking shelter, running low on food and largely drinking contaminated water.

An airstrike hit a residential building some 200 metres from the UN headquarters in Rafah on Monday, killing and wounding several people, according to an Associated Press reporter at the scene, underscoring the perils of humanitarian operations.

After a day of intense strikes, Rafah's Abou Youssef Al-Najjar Hospital registered 61 deaths since Monday morning, its spokesperson said. With no room in the morgue, more half the bodies were lying in the hospital grounds, spokesperson Talaat Barghout said.

At least 1.4 million Palestinians in Gaza have fled their homes, and nearly 580,000 of them are sheltering in UN-run schools and shelters, the UN said Monday.

No aid will be distributed in Gaza City and other parts of the north, where hundreds of thousands of people remain. Gaza City's main al-Shifa Hospital, with a normal capacity of 700 patients, is currently overwhelmed with 5,000 patients, and around 45,000 displaced people are gathered in and around its grounds for shelter, the U.N. said.

“The north didn't receive anything” from incoming aid, said Mahmoud Shalabi, an aid worker with Medical Aid for Palestinians aid group based in the northern town of Beit Lahia. “It's like a death sentence for the people in the north of Gaza.”