The Israeli military says thousands of its troops have surrounded an underground Hezbollah fortress beneath the hills of southern Lebanon, trapping dozens of militants inside.

The standoff is at the heart of intense clashes that have erupted in the area in recent days between U.S.-allied Israel and Iran-backed Hezbollah.

The clashes have already threatened to upend delicate progress toward a framework peace agreement between Tehran and Washington. Should the confrontation end in a blood bath, it could disrupt those negotiations again.

Despite an Israel-Hezbollah cease-fire and even after a halt to out-and-out combat over the weekend, fighting has persisted in the hills this week, particularly on a strategic ridge called Ali al-Taher, just north of the town of Kfar Tebnit.

On Tuesday, Israeli troops fired on what the military said were Hezbollah operatives in two separate actions in the area, killing at least two people. Hezbollah said that those killed were civilians.

Israeli officials say they believe that the trapped Hezbollah fighters are running out of food and water.

Hezbollah has built one of its largest underground facilities beneath the Ali al-Taher ridge, which is about six miles from the border with Israel, according to two senior Lebanese officials who requested anonymity because of the sensitivity of the matter.

Israel says the complex has long been used to orchestrate and launch attacks against its territory.

Hezbollah did not reply to a request for comment.

The tunnel complex is about three miles north of Beaufort, an Crusader castle on a strategic hilltop that Israel said it captured on May 31.

While the seizure of Beaufort was seen as largely symbolic at the time, the Israeli military says that it was merely the first step in a ground advance that required a full month and whose ultimate objective was capturing the underground installation near Kfar Tebnit.

Israeli experts say the tunnel complex took more than 20 years to build — with Iranian assistance — and is the headquarters of Hezbollah’s Badr Unit, which is considered part of the group’s forward line of operations against Israel. The unit has been one of Hezbollah’s main forces in southern Lebanon since 2024.

“From this place, you can launch missiles and munitions at Israel,” said Sarit Zehavi, president of the Alma Research and Education Center, which focuses on security challenges on Israel’s northern border. “It’s 10 kilometers from Metula,” an Israeli border town, she added.

Israel occupied the ridgeline after its 1982 invasion of Lebanon, and it was among the last positions that Israel vacated during its withdrawal from southern Lebanon in 2000. Much of the rolling landscape in the area now is covered in bunkers and underground facilities built by Hezbollah, according to one of the Lebanese officials.

Security analysts say that Hezbollah was able to build and maintain that infrastructure because the area lies just north of the Litani River, outside the longstanding mandate of the U.N. peacekeeping force in Lebanon.

The area was also outside the scope of the Lebanese Army’s efforts to disarm Hezbollah after the 2024 cease-fire, which focused on the area south of the Litani.

Israel and Hezbollah have fought two wars in the past three years.

Satellite images show heavy strikes since May on the Ali al-Taher ridge, including in areas where historical satellite imagery showed construction work in recent years. The recent images also show extensive destruction to the town at the base of the hill.

As recently as May, the Ali al-Taher ridge was outside the so-called buffer zone that Israel had established in Lebanon several miles north of the border. Israel delineates the buffer zone with a yellow line on a map, and says it considers any armed fighter south of the line a threat and thus a legitimate target.

But Israel redrew the yellow line demarcating the buffer zone on June 18, specifically extending it to encompass Ali al-Taher.

A day later, fighting between Israel and Hezbollah became intense in the area after four Israeli soldiers, one of them a battalion commander, were killed when their tank exploded. Israel responded with waves of airstrikes.

An Israeli military spokeswoman said the army believed that the fierceness of Hezbollah’s fire as troops drew near the underground complex was proof of the site’s importance to the group.

Israeli officials say that thousands of soldiers are surrounding the complex.

“We assume they are trapped and figuring out what to do,” the military spokeswoman said of the Hezbollah fighters, “whether to battle us, or surrender, or wait for something to change — either because of the cease-fire or if there’s an Israeli withdrawal.”

The situation bore similarities to a standoff in the Gaza Strip last fall, where hundreds of Hamas fighters were caught in tunnels behind Israeli lines after a cease-fire was reached.

Israel demanded their unconditional surrender, but Hamas pressed for their safe passage. Many ultimately surrendered or were killed, the military said.