SABRA LANE, PRESENTER: Australian warplanes have now flown more than a hundred missions over Iraq as part of the global campaign against Islamic State.

But Iraq and Syria aren't the only targets of this lethal terrorist group.

Lebanon is also in its sights and the threat to the Lebanese is becoming more serious after several recent militant attacks on its territory.

Across the nation, citizens are arming themselves in preparation for any IS advance and the Shiite militia Hezbollah is bolstering its forces near the Syrian border.

Middle East correspondent Hayden Cooper reports from Lebanon.

HAYDEN COOPER, REPORTER: Lebanon's capital, Beirut: a city with a violent past and an uncertain future, where relics of civil war still stand and Christian, Shiite and Sunni live side by side.

In a region of chaos and killing, this is a pocket of relative calm. But a menacing force is amassing at the border: the black-clad bandits of the Islamic State.

HAMZA AKL HAMIEH, FORMER HEAD OF THE AMAL MILITIA (voiceover translation): We are expecting the worse right now in Lebanon, but the Government and the people are ready to fight this terrorism.

HAYDEN COOPER: In the shadow of the nation's parliament, distraught Lebanese citizens are staging a protest. These are the relatives of more than 30 soldiers and police who have been captured by two lethal terrorist groups: the Islamic State and the al-Qaeda-linked Jabhat al-Nusra.

Seized near the Syrian border in the town of Assal, three of the group have already been executed. Nawfal Al-Sheikh shows me a hostage video in which his captured brother appears.

NAWFAL AL-SHEIKH, BROTHER OF HOSTAGE (voiceover translation): It's most difficult for his wife. She gave birth a week ago and she was very sad. There's just misery and no happiness.

HAYDEN COOPER: This is the moment they were taken: August this year when Islamic State and Jabhat al-Nusra fighters attacked Assal together. They briefly overran the town before being forced back and taking the captured soldiers and police with them.

LOCAL MAN (voiceover translation): No matter what I say, I can't explain to you what we are going through right now.

HAYDEN COOPER: All of this has happened only a short distance from Beirut, an hour's drive over the mountains and into the fertile Beqaa Valley, a place that's now riven with tension from Syria's civil war just a few kilometres away.

This is basically the front line in Lebanon's battle to keep the extremists out. In the mountain range behind me is the border with Syria and the fighters of Islamic State and Jabhat al-Nusra. Here in the valley, hundreds of thousands of Syrian refugees and a constant fear that the militants want to extend their so-called caliphate into Lebanon.

Lebanese villagers are preparing for the worst, weapons at the ready.

It's easy to find groups of farmers who have already leapt into action once to repel the attacks by Islamic State fighters. This group fought them in a second cross-border raid in October on the nearby town of Brital.

AREF HAMIEH, FARMER (voiceover translation): We left with about 3,000 people from all over the valley, even more than that. Here, we live as tribes and we are united. They are like a foreign virus.

HAYDEN COOPER: All Shiites, the villagers point out how close the attack came when IS and Nusra fighters approached the outskirts of Brital.

AREF HAMIEH: Flee to Syria.

HAYDEN COOPER: They went back to Syria?

AREF HAMIEH: (Nods assent).

HAYDEN COOPER: Despite conceding their opponents are well-armed with stolen American and Iraqi weapons, these men are dismissive of the Islamic State's capabilities.

AREF HAMIEH: (voiceover translation): They have no technique, no thinking at all. They are from the Stone Age. They're a bunch of druggies, a bunch of drunks and they can't do anything to us. We are the capital of the resistance.

HAYDEN COOPER: Any mention of a caliphate is met with scorn.

QASSEM TLAIS, FARMER (voiceover translation): (Laughter) Hilarious. Hilarious. This is a joke. A complete joke.

HAYDEN COOPER: There is a reason for their supreme confidence. They're backed by the Shiite military force of Hezbollah.

We've arranged a rare interview with a Hezbollah commander. Their fighters are banned from speaking publicly.

This man tells me he has killed dozens of Islamic State and Nusra militants himself.

HEZBOLLAH COMMANDER (voiceover translation): In Brital, they put all their strength into the attack and took only one Hezbollah position. But they could only hold it for an hour and 20 minutes.

HAYDEN COOPER: Hezbollah is fighting on the side of the Syrian regime. It's secured major victories against Syrian rebels in border towns like Qusayr and Homs. It takes its orders from Iran, which wants to maintain the Shiite crescent stretching from Tehran through its allies in Baghdad, Damascus and into Lebanon. It allows a steady flow of weapons to Hezbollah.

Long designated terrorists by the West, the group is now an unlikely ally in the campaign against the Islamic State. Even so, this fighter still considers Australia an enemy.

HEZBOLLAH COMMANDER (voiceover translation): Not the people of Australia, the regime of Australia who support Israel and the Americans. As a Hezbollah fighter, I salute the Australian people, but not the Government. In reality, they are bombing the pickup trucks of Islamic State, not Islamic State themselves.

HAYDEN COOPER: There are many Australians who are fighting with the Islamic State. What do you, as a Hezbollah fighter, have to say to them?

Hezbollah commander (voiceover translation): God willing, their punishment awaits them.

HAYDEN COOPER: Sectarian conflict is nothing new to Lebanon. Its own civil war lasted 15 years.

Veteran leader Hamza akl Hamieh is the former head of the Shiite Amal militia.

HAMZA AKL HAMIEH, FORMER HEAD OF THE AMAL MILITIA: We are fighting for humanity. The terrorism is against all humankinds.

HAYDEN COOPER: For the families of the Lebanese security forces captured in Assal, life itself has changed dramatically. Nervously they wait for any news of a breakthrough in negotiations.

LOCAL MAN (voiceover translation): We are living this every second that goes by. Even now, I'm sitting here with you, but my heart is there with them. They are facing execution any second.

SABRA LANE: Hayden Cooper reporting.