For a month, several small groups of Hizbullah militants on a Lebanese hillside have withstood heavy artillery shelling and airstrikes to continue firing hundreds of Katyusha rockets into Israel from positions just a few hundred yards from the border.

Even seasoned observers with the United Nations peacekeeping force in south Lebanon, known as UNIFIL, whose headquarters lies at the foot of the hillside, are baffled at how the guerrillas have managed to survive and keep up their steady rocket fire.

It's just one example of Hizbullah's surprising resilience in this war. Their ability to continue fighting against the most advanced and powerful army in the Middle East is rooted in the group's meticulous planning and thorough understanding of Israeli military doctrine and capabilities.

"They have done incredible staff work, learning the lessons of guerrilla warfare down the ages and carrying out a very deep and accurate analysis of the Israeli army," says Timur Goksel, who served with UNIFIL from 1979 to 2003 and witnessed Hizbullah's military evolution over two decades.

Israeli forces are poised to mount a full-scale invasion of south Lebanon – pending a last-ditch negotiation effort – in a bid to crush their Hizbullah foes and drive the remnants north of the Litani river, about 18 miles north of the border. Still, despite two decades of experience fighting Hizbullah in south Lebanon, the Israeli military appears to have underestimated the resilience of their Lebanese opponents.

Using advanced anti-armor missiles, snipers, and roadside bombs, and operating from an complex system of bunkers and tunnels, Hizbullah's battle-hardened fighters have survived airstrikes and artillery barrages enabling them to keep killing Israeli soldiers and firing rockets. On Wednesday, 15 Israeli soldiers were killed, the highest toll in a single day since the war began.

"They have lots of strongholds, which are very well disguised, and we need to eliminate their ability to attack Israel from these places," says a senior IDF military official. "It's a very hilly area and it's not easy. You cannot identify their bunkers until you're right there."

Small forces of locals

The militants presently on the front line are thought to number no more than 1,000, a fraction of the potential force Hizbullah could unleash. The fighters are drawn from the villages on the front lines, using their intimate knowledge of the local terrain to their advantage. Local groups of Hizbullah fighters communicate with each other by walkie-talkie using a code that draws upon their personal knowledge of each other and the geography. If Israeli forces push deeper into Lebanon, moving to new towns and villages, they will encounter new lines of fresh Hizbullah combatants.

"Even I have been surprised at the tenacity of these groups fighting in the villages. They have fought far beyond my expectations. And they haven't even committed all their fully-experienced troops yet," says Mr. Goksel.

The guerrillas are drawing on years of meticulous preparations and training, combined with access to newer weapons technology. The most effective weapons system employed by Hizbullah's front line guerrillas are antitank missiles. Small teams of specially-trained fighters have inflicted comparatively heavy casualties on Israeli troops, using advanced missiles to knock out the formidable Merkava tank and using older versions to punch through the walls of houses sheltering Israeli soldiers.

In the current war, Hizbullah has used for the first time the Russian Metis-M, which can be fitted with an anti-armor warhead for destroying tanks, or a fuel-air explosive warhead to use against troops or bunkers. The missile has a range of about one mile. Hizbullah is also reported to be employing the laser-guided Kornet-E anti-tank missile which has a range of about 3.5 miles. Individual fighters carry the shoulder-fired RPG-29, a more advanced version of the RPG-7 loved by guerrilla groups around the world since the 1960s. The RPG-29 was first used by Hizbullah last November in a failed attempt to kidnap Israeli soldiers.

The ability of the well-trained missile teams to knock out Israel's vaunted Merkava tanks has frustrated the traditional Israeli military doctrine for rapid armored thrusts deep into enemy territory. Instead, Israeli forces have inched cautiously northward and even after more than two weeks of stiff fighting have yet to capture and secure key border towns.

Was Hizbullah underestimated?

There's been criticism in Israel that the military waited too long to move against Hizbullah with large forces of infantry and armored vehicles.

"I don't think Hizbullah was underestimated by a great deal, but there was an expectation that the air attacks would cut off the command and control process,'' says Gerald Steinberg, political science professor at Bar Ilan University outside Tel Aviv. "There was a hope that this could be done without large scale casualties.''

In an indication of the difficulty, Israeli Chief of Staff Dan Halutz sidelined the general charged with overseeing the ground war in Lebanon. The decision was the first time since the 1973 Arab-Israeli war that a general has had his authority curtailed in the middle of fighting.

"As a fighting army, I don't find the IDF performing worse than it has in the past,'' said Yossi Alpher, the editor of the online Arab-Israeli journal Bitterlemons.org. "These are probably the best trained and most highly-motivated Arab fighters we've ever faced.''

Mr. Alpher noted that Israel's army has never succeeded in snuffing out rocket fire from Lebanon, going all the way back to the 1982 invasion.

If Israeli troops enter Lebanon in strength, Hizbullah will resort to another favorite weapon, the roadside bomb. Roadside bombs killed more Israeli soldiers in south Lebanon in the 1990s than any other weapon, and the technology used to build and detonate them has become highly sophisticated.

Early roadside bombs consisted of home-made Claymore-style explosive charges that spray hundreds of ball bearings, and were detonated by a command wire or remote radio control.

Hizbullah's bombs today include shaped-charge warheads which concentrate the blast in a single direction, similar to an anti-tank missile, to punch through the walls of armored vehicles. They are detonated by infrared beam – when the beam is broken, the bomb explodes.

Military observers believe that Hizbullah long ago planted huge mines under all the roads crossing the border. In June 2002, a shepherd inadvertently stumbled across one of these bombs and alerted the local police, much to Hizbullah's annoyance. It consisted of 180 kilograms of plastic explosive that had been buried several feet beneath a road 500 yards from the border.

In anticipation of these land mines, Israeli tanks have avoided the border roads, cutting across country instead.

Secret, intricate bunker system

From the summer of 2000, Hizbullah established an intricate and secret military infrastructure in south Lebanon, consisting of tunnels, expanded natural caves, and underground bunkers where weapons were stored and fighters could live. Much of this construction work was carried out in remote stretches of the border away from the public gaze and at night. Three years ago, residents of villages near a valley known to be controlled by Hizbullah were kept awake at night by the sound of explosions as the guerrillas dynamited the limestone cliffs to build new bunkers.

Highly trained Hizbullah marksmen have also contributed to Israel's troop casualties. Equipped with high-powered rifles, the snipers lie undercover for days at a time, picking off Israeli soldiers when the opportunity arises. In July 2004, a Hizbullah sniper shot dead with extreme precision two Israeli soldiers from a range of 500 yards.

Israeli commanders concede that while they claim to have destroyed many of Hizbullah's long-range rocket launchers, including the 600mm Zelzal which can reach Tel Aviv, the standard 122mm Katyushas can be fired more easily by mobile teams without the need for launchers visible to spotter drones or surveillance planes. Generally, the rockets are fired from multi-barreled launchers on the back of flat-bed trucks, but they can also be fired singly, even from a simple mounting of crossed sticks which is all but invisible to drones and aircraft when hidden inside an olive grove or orange orchard. Some rockets are fired by timers, allowing the militants to escape the area in advance.

Despite saturation air coverage with missile-firing reconnaissance drones, F-16 fighter-bombers and Apache helicopter gunships, the Israelis have been unable to stem the flow of rocket fire across the border. Last week, Israeli commandos staged a pre-dawn raid on an apartment block on the outskirts of Tyre housing a team of Hizbullah militants who had been firing long-range rockets into Israel. But hours after the raid, rockets were again being fired from the same location.

Weapons caches small, dispersed

Instead of stockpiling munitions in just a handful of arsenals, Hizbullah carefully dispersed ammunition, rockets, and weapons all over south Lebanon, stashing them in private homes, garages, basements, bunkers, and caves. That has ensured that small isolated Hizbullah units have a readily available supply of weapons and ammunition.

"They have amassed huge stockpiles of rockets in the area," says Gen. Alain Pellegrini, UNIFIL's commander. "I think the Israelis were hoping they would have had a faster success against Hizbullah by now."

The most important weapon in Hizbullah's arsenal may in fact be the motivation and determination of each fighter.

"The number one element is that Hizbullah is not afraid of the Israelis," says Goksel. "After 18 years fighting Israeli troops, they see them as vulnerable human beings who make mistakes and are afraid like anyone else."

• Staff writer Ilene Prusher in Jerusalem and correspondent Josh Mitnick in Tel Aviv contributed to this report.