The Israeli military said it had recovered the bodies of more than 1,500 Hamas fighters inside Israel and mostly secured its border with Gaza after a night of intensified airstrikes across the enclave that destroyed infrastructure and displaced thousands. An Israeli military spokesperson said its forces were installing “an iron wall” of tanks and helicopters along the Gaza border, ahead of an expected large-scale ground invasion of the besieged enclave. The UN agency for Palestinian refugees said that more than 187,000 people had been displaced across the Gaza Strip, with that number expected to increase. A spokesperson for the Israeli forces, Lt-Col Richard Hecht, advised Palestinians to “get out” of Gaza through the Rafah crossing with Egypt on its southern border, before clarifying that the crossing remained closed. Images online showed damage from an airstrike to the crossing on Monday. “Around 1,500 bodies of Hamas (fighters) have been found in Israel around the Gaza Strip,” Hecht said, adding that security forces had “more or less restored control over the border”. Ali Barakeh, a member of Hamas’s exiled leadership in Beirut, told the Associated Press that the group was prepared for a prolonged war. “We have prepared well for this war and to deal with all scenarios, even the scenario of the long war,” he said, adding that Hamas intended to exchange Israeli hostages held in Gaza for Palestinians held in Israeli jails, and even some in the US. Barakeh’s comments followed statements by another senior Hamas official, Moussa Abu Marzouk, who told Al Jazeera that the group was open to discussions of a possible truce with Israel after having “achieved its targets”. In a telephone interview, Abu Marzouk said Hamas was open to “something of that sort” and “all political dialogues” regarding the group’s willingness to discuss a potential ceasefire. His comments came as concerns grew about the fate of more than 100 individuals held by militants in Gaza. On Monday, Hamas threatened to start executing its hostages if Israel carried out airstrikes in the Gaza Strip without prior warning to residents. The Qatari foreign ministry said on Monday that it was in mediation talks with Hamas and Israeli officials, including over a potential prisoner swap, although neither party confirmed the talks. Palestinian militants abducted more than 100 people during a surprise multi-front attack in which they killed more than 700 – making Saturday the deadliest day in Israel’s history. Israeli media said on Monday the death toll had climbed to 900. In response to the attack, Israel has launched strikes from the air and sea. The Palestinian health ministry said the death toll in Gaza had climbed to more than 700, including 140 children, with thousands wounded. It called for a “safe corridor” to allow the passage of aid into Gazan hospitals. Hecht warned that Israeli airstrikes might not have the same “level of fidelity” in warning targets in Gaza before striking, amid increasing reports of bombardments on residential buildings and civilian infrastructure. The Gaza Strip is home to at least 2.3 million people, an area considered one of the most densely populated places on Earth. Israeli officials declared a “complete siege” of Gaza, cutting off water, food and power supplies. The intensifying siege follows Israel’s 16-year blockade of Gaza. There are rising fears of a potential communications blackout for those trapped inside, due to electricity and internet cuts. The Palestinian news agency Wafa said at least two Palestinian journalists had been killed in an airstrike early on Tuesday morning. Israel formally declared war on Sunday and called up 300,000 reservists for duty, signalling a possible ground assault into Gaza – a move that in the past has always brought further bloodshed. Israeli forces face the unprecedented task of fighting an urban war while dozens of hostages are likely to be hidden in tunnels and basements across the Gaza Strip. Mairav Zonszein, of the International Crisis Group, said: “This is clearly preparation for a major ground invasion. I don’t think there’s any basis in understanding how that will either bring back the captives or get rid of Hamas. “The only thing I can see from this is full-on vengeance, exacting a price for the heavy price Israel is paying now.” Zonszein said the Israeli prime minister, Benjamin Netanyahu, had not publicly prioritised the safe return of the hostages held by militants from Hamas and the Palestinian Islamic Jihad, despite the threats of executions. “Netanyahu’s speech yesterday didn’t say anything about returning captives; I don’t think that’s a priority,” she said. In his interview with the Associated Press, Barakeh said that only a small number of top commanders inside Gaza had known about Saturday’s incursion into Israel and that even the group’s closest allies had not been informed in advance about the timing. He denied reports that Iranian security officials had helped plan the attack. However, he added that allies such as Iran and the Lebanese Hezbollah “will join the battle if Gaza is subjected to a war of annihilation”. He said even Hamas was shocked by the extent of the operation, saying it had expected Israel to prevent or limit the attack. “We were surprised by this great collapse,” Barakeh said. “We were planning to make some gains and take prisoners to exchange them. This army was a paper tiger.” In a televised address late on Monday, Netanyahu pledged to “eliminate terrorists” still present in Israel and thanked the US for its “unequivocal support”. The World Health Organization said there had been 11 attacks on healthcare sites – which included medical facilities, ambulances and care providers – in the first 36 hours of the new conflict in Gaza. “There is an urgent need to establish a humanitarian corridor for unimpeded, life-saving patient referrals and movement of humanitarian personnel and essential health supplies,” the WHO said. The US confirmed that the first in a batch of military aid was “making its way” to Israel. “We fully expect there will be additional requests for security assistance for Israel as they continue to expend munitions in this fight,” the White House national security spokesperson, John Kirby, said. “We will stay in lockstep with them, making sure that we’re filling their needs as best we can and as fast as we can.” The US has not yet detailed the extent of Israel’s requests for security assistance, but a defence official told the Associated Press that Washington would expedite pending Israeli orders and look to the US military’s own stockpiles to help fill Israeli gaps. “There should be an international call for an immediate cessation of hostilities,” said Zonszein. “But the US isn’t doing that, it’s not calling on Israel to stop. Biden is giving Israel military and diplomatic support.”