By Nicholas BlanfordThe Daily StarZRARIEH, Lebanon: Although Lebanon’s northern border with Syria has grabbed most international and domestic attention lately, the discovery of another Israeli tap on Hezbollah’s private communications network is a reminder that the intelligence war in south Lebanon continues uninterrupted.Hezbollah has been characteristically reticent in describing how the existence and location of the tap on the fiber-optic cable near Zrarieh was determined, despite welcoming the media to inspect the scene of Israel’s latest violation of Lebanese sovereignty.

But it is likely the tap was discovered using similar means that led to the unearthing of previous Israeli interceptions near Srifa in December last year, and south of Houla in October 2009.

There are different ways of tapping fiber-optic cables, but one of the more common methods is to clamp the cable to create micro-bends that allow data-conveying photons to bleed out. Although the amount of light lost from the cable is minimal, it is sufficient to permit the eavesdropper to interpret the data conveyed by the cable. There are also passive means of tapping fiber-optic cables, which do not require a direct physical tap, making them harder to detect.

Similarly, there are various methods to protect fiber-optic cables from interception or to detect existing taps. Intrusion detection systems use optical power monitors, which measure minute fluctuations in the power level suggesting a possible leakage of data via a tap, and optical time domain reflectometers, which can pinpoint the location of a breach along the cable.

Hezbollah most likely uses such equipment to detect and locate Israeli taps on its communications network. In October 2009, a team of Hezbollah engineers narrowed down a suspected tap to a valley just south of Houla.

The team dug holes every few meters to inspect the cable, all the while tailed overhead by an Israeli reconnaissance drone. Upon discovery of the monitoring device – consisting of an interceptor attached to the fiber-optic cable, a connected radio transmitter buried about 10 meters away and a power pack consisting of 360 batteries – the Hezbollah team backed away and the Israelis attempted to destroy the device using the drone to relay an electronic signal to detonate explosives attached to the three components.

Only the transmitter blew up, however, allowing UNIFIL and the Lebanese Army, which were alerted by the explosion, to inspect the equipment. The Israelis successfully destroyed the remaining components in a second blast a day later.

Perhaps the more puzzling question is less the means by which Hezbollah discovered the tap and more how the Israelis were able to plant the device in the first place.

Judging from photographs and TV footage from the scene of the incident on a hillside near Zrarieh, the equipment was bulky and included several components, among them an 80-meter cable, according to Al-Manar television.

The tapping device outside Houla is thought to have been planted during the July 2006 war when Israeli troops were in control of the area. Similarly, a disguised Israeli surveillance device consisting of a camera and transmitter hidden near Shamaa in the southern border district which was discovered in March last year also could have been planted during the 2006 war.

But in 2006, the Israelis never reached Zrarieh, nor Srifa for that matter, where another tapping device was installed before being discovered seven months ago. Lugging heavy equipment, digging holes deep enough to bury it and establishing the tap itself suggests the work of a team skilled in covert activities and with the required technical expertise.

That makes it unlikely that the tapping device was installed by a local Lebanese collaborator.

In December 2010, the Lebanese Army recovered camouflaged surveillance and monitoring equipment from the Barouk and Sannine mountains. (Hezbollah’s technicians discovered the devices and passed on the information to the Army).

The Barouk and Sannine areas are remote, unpopulated and lie outside Hezbollah’s geographical control, making it easier to install equipment without anyone noticing. But the heart of south Lebanon is entirely different. Was it possible for a team of Israeli agents or special forces soldiers to infiltrate an area where Hezbollah is omnipresent and plant the sophisticated and bulky devices before departing unseen?

Hezbollah may have developed the technical expertise to trace interceptions on its communications network, but the ability of Israeli agents, or whomever, to plant the devices undetected in the first place suggests that the more serious security breach is in Hezbollah’s failure to protect its operational environment.

Meanwhile, the redeployment of at least one battalion of Lebanese troops from the southern border district because of security disturbances elsewhere in the country has led to an increase in “denials of access” to UNIFIL patrols, according to sources in the peacekeeping force.

UNIFIL often mounts joint patrols with Lebanese soldiers, which generally ensures there is no friction with local residents. But deteriorating security in the north and Beirut in recent weeks has led to the redeployment of one Lebanese Army battalion and possibly an additional three company-sized units drawn from each of the three Army brigades deployed in the south, according to security sources.

The reduction in numbers of Lebanese soldiers has led to an increase in UNIFIL-only patrols. Even though each patrol is coordinated in advance with the Lebanese Army, UNIFIL sources say there has been an uptick in incidents where the peacekeepers are denied access to certain areas by what they believe is a combination of civilians and Hezbollah personnel.

A patrol of Irish soldiers was challenged Saturday by a group of residents in Maroun al-Ras near Bint Jbeil when the peacekeepers were spotted taking photographs. Although the photographs were apparently for personal use rather than information-gathering, the soldier chose to hand over the camera to the residents.

UNIFIL played down the incident as an isolated event. But UNIFIL sources say there have been as many as one or two “denials of access” almost every day over the past six weeks spread across the peacekeepers’ area of operations and not targeting any one contingent.

The sources added that there was nothing threatening in the confrontations and assess that it is a “question of local actors reminding us who’s in charge down here.”