BEIRUT — After a series of staggering losses, Hezbollah is putting up a stiff fight against Israeli forces in Lebanon’s south while continuing to rain down rockets across the border, underscoring the group’s resilience and the limitations of Israel’s ground campaign.

When Israel sent troops across the border on Oct. 1, officials estimated military operations would last for a few weeks. More than three weeks later, officials have said they will likely need a few weeks longer, raising concerns over the kind of mission creep that has defined Israel’s past wars in Lebanon. A resurgent Hezbollah damaged Prime Minister Benjamin Netanyahu’s home in a drone attack over the weekend and has warned a “new escalatory phase” is coming.

The militant group has bounced back from its unprecedented setbacks — including the penetration of its electronic devices and the assassination of most of its senior leadership — thanks to a flexible command structure, help from Iran and years of planning for an Israeli invasion, current and former Lebanese officials said. Israeli officials, for their part, contend that the group remains significantly weakened and operations on the ground in Lebanon are going according to plan.

Mohammed Afif, head of Hezbollah’s media office, told reporters Tuesday he wanted to speak directly to “the enemies of the resistance.”

“For a moment, you thought that it was over,” he said, but “you misunderstood us ... Hezbollah was able to rehabilitate its capabilities and a great portion of our abilities remain in the field.”

For all the bluster, the area where he held the news conference — in Beirut’s southern suburbs, the group’s longtime seat of power — has been decimated. Wave after wave of Israeli airstrikes have flattened apartment blocks and displaced tens of thousands of residents. Shops have been boarded up against looters.

Soon after the news conference concluded, an Israeli strike leveled another mid-rise apartment building just a few hundred yards away. The next day, Hezbollah confirmed the death several weeks earlier of Hashem Safieddine, the man widely believed to have been in line to replace its former leader, Hasan Nasrallah, assassinated by Israel last month.

In the south, though, Hezbollah is holding its own, with Israeli forces advancing no more than four or five kilometers into Lebanon at their farthest point, according to a Lebanese official. Like others in this story he spoke on the condition of anonymity to discuss sensitive matters.

“They are a formidable foe,” said an official with the Israel Defense Forces, one of four Israeli security officials who spoke with The Post about the state of the fight in Lebanon. The official said Hezbollah militants are better trained, more experienced after fighting in Syria and armed with more advanced weaponry than in 2006, during their last war with Israel.

Hezbollah has “transformed itself from a terror organization to a terror army,” the IDF official said.

Israel continues to describe its invasion of Lebanon as “limited,” aimed at allowing tens of thousands of displaced residents to return to their homes in the north — and to protect them from the kind of devastating cross-border attack that Hamas launched from Gaza last October. But if Hezbollah is able to reconstitute, Israel could find itself drawn into a much longer and more costly conflict.

Nineteen Israeli soldiers have been killed so far in Lebanon, the IDF says. Estimates of Hezbollah losses range from hundreds of fighters to more than 1,000, according to two diplomats who have been briefed on Israeli assessments of the war. But the group has given no signal it plans to retreat.

Former and current Lebanese officials say the group has regained its footing in large part because it is operating on familiar turf in the south, where it can execute guerilla-style attacks.

Hezbollah operatives are using the rugged hillsides and steep valleys to their advantage, the officials say, which Israel is trying to counteract with clearing operations — demolishing homes, tunnels, bunkers and razing vegetation as their forces advance.

Hezbollah’s ground forces also maintain independent command structures and communication networks, according to Lebanese officials. That system insulated front-line fighters and commanders from the pager attacks last month that left many senior and mid-ranking Hezbollah members in Beirut unable to pass even simple messages to each other.

“Hezbollah is like a Russian doll,” said one Lebanese official who meets regularly with the group’s political leadership. “You think it’s one thing, but when you open, it’s many, all independent.”

A Lebanese individual close to Hezbollah said the “decentralized” operations were adopted after the 2006 war, when most coordination came from the Lebanese capital. The autonomy of forces in the south has allowed the fight to progress, the official said, but it has also left Hezbollah “fragmented” — simultaneously capable of taking on the region’s most advanced army while unable to carry out basic functions.

Hezbollah never held a funeral for Nasrallah, a decision one Arab diplomat in Beirut attributed to the group’s ongoing inability to plan and communicate. Hezbollah officials have also struggled to meet with foreign diplomats in person after many of the group’s offices were taken out by airstrikes, according to two Arab diplomats.

But Iran, Hezbollah’s main patron, has stepped up its support of the group over the last month, sending additional supplies, munitions and personnel to Lebanon through Syria, said a Lebanese official from a Shiite party allied with Hezbollah.

“Iranian support for Hezbollah on the battlefield has intensified,” the official said, pointing to a “notable increase” in on-the-ground advisers. He emphasized that Iran was not directly participating in the fighting.

Israeli commanders remain bullish on the war’s outlook, saying they have already managed to destroy a large portion of the group’s arsenal and are well on their way to dramatically degrading the group’s military capabilities. Yet Hezbollah’s rocket and drone barrages toward Israel have actually increased over the last three weeks — from about 100 projectiles a day at the start of ground operations to around 150 in recent days, according to IDF statements.

But the group still has yet to unveil the larger, longer-range — and potentially more deadly — munitions it is believed to possess. The vast majority of the rockets launched into Israel are of the low-grade, unguided variety, and most are easily intercepted by Israeli air defenses.

Israeli officials believe their attacks on Hezbollah weapons stores and launch capabilities have prevented the group from unleashing waves of larger missiles deep into Israel — the nightmare scenario the country had long braced for.

“We’ve been surprised that we are operating in southern Lebanon and that our people’s routines in Haifa and Tel Aviv have not been disrupted,” a second IDF official said. “That’s not what we expected.”

Some analysts said Israel’s relief may be premature. “Hezbollah may resort to more advanced missile systems in greater numbers and frequency if the war escalates,” Nicholas Blanford, a Hezbollah expert at the Atlantic Council, wrote in his assessment of the conflict.

“It is in Hezbollah’s nature to act incrementally when confronting the Israeli military, reserving some of its more advanced weaponry for when it is needed and to retain the element of surprise,” he said.

Hezbollah’s most successful attacks on Israeli territory have been carried out with drones, which are cheap and readily replaceable. It was a Hezbollah suicide drone that killed four Israeli soldiers on a military base in central Israel on Oct. 13. And it was a drone that struck Netanyahu’s private residence on Saturday in Caesarea, more than 50 miles from the Lebanese border. Netanyahu was not at home at the time of the attack.

The unmanned devices are often small; some are electric, leave no heat trace and are able to fly low, avoiding Israel’s vaunted air defense systems. The IDF, like militaries around the world, are scrambling to adapt to this new form of aerial warfare.

“We are focused now on jump-starting our capacity and we think in a few months we’ll be in a very different place,” the second IDF official said.

Afif, the Hezbollah media chief, said Tuesday that attacks on Israel would only intensify. Then he addressed Netanyahu directly.

“If we didn’t reach you with our hands this last time, then between you and us is days, nights and the battlefield,” he said.

Hendrix reported from Jerusalem.