Syria's civil war has brought an end to decades of calm on the Golan Heights, a strip of land which Israel captured in the 1967 Middle East war. Battles between rebels fighting against President Bashar Assad's forces in Syrian villages nearby are being watched intensely by Israel's military.
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Hezbollah, which is also backed by Iran, has sent thousands of its own fighters to combat Syrian rebels, according to Israeli and Western estimates.
Israel last fought Hezbollah in a 2006 Lebanon war and still closely monitors the Lebanese border. Israel says Hezbollah has tens of thousands of rockets in its south Lebanon stronghold.
The Jewish state is worried Hezbollah is making initial preparations for future confrontation with it on a new front with Syria and is accruing valuable combat experience on the Syrian battlefield.
An Israeli source said the group is gathering intelligence on Israel's deployment on the strategic Golan plateau.
"It is not at an alarming level now but we understand their intentions," said the source, who asked not to be identified because of the sensitivity of the security and political situation in the area.
Car bomb explosion in Beirut, Tuesday (Photo: EPA)
Hezbollah's leader, Hassan Nasrallah, threatened in May to turn the Golan into a new front against Israel.
"Since Nasrallah's threat, more (Israeli) army companies have been sent up, more tanks," an Israeli military source at the Booster military outpost on the Golan.
"Hezbollah has an intelligence presence (in the Golan) that we know of."
Booster is about two kilometers (one mile) from a disengagement line set after Israel and Syria fought on the Golan in 1973 and Israeli tanks have just moved back into the position for the first time since then.
Daytime is peaceful on the rocky outcrop that gives a turret-top view of Syrian villages below, with birdsong echoing across sun-scorched fields. That changes at nightfall.
"Every night there is fighting (in the villages across the frontier), explosions and shooting all through the night. This is the hottest spot on the Golan Heights," Shilo said. "As far as we're concerned, any bullet that crosses over is intentional."
Nasrallah (L) with Assad and Ahmadinejad (Archive photo: AP)
A UN observer force monitors the area of separation between Syrian and Israeli forces, a narrow strip of land running 70 km (45 miles) from Mount Hermon on the Lebanese border to the Yarmouk River frontier with Jordan.
The observers have been caught in the middle of fighting between Syrian troops and rebels. Stray shells and bullets have landed on the Israeli-controlled side of the Golan, and Israeli troops have fired into Syria in response.
Beirut residents celebrate Hezbollah success in Syria
The rebels have detained peacekeepers on several different occasions before releasing them. Japan and Croatia have withdrawn troops due to the violence as has Austria with the gap being filled by soldiers from Fiji.
Among the rebels fighting the Syrian army are jihadi and Qaeda-linked groups, which Israel says are also a future threat to the Jewish state.
"We know they are busy now but once it ends they will turn their guns on us," said the military source.
"We have learnt our lessons from Sinai," the source said, referring to the Egyptian peninsula where Islamist militants have launched attacks on Egyptian soldiers and across the border at Israel amid deepening turmoil in Egypt.
"We're not waiting for an attack (from Syria). We're building the border fence, we have sent up tanks, more regiments, field intelligence ... and increased observations."
Israel is particularly worried that Hezbollah will get hold of advanced weapon systems or chemical arms in Syria. Israel has struck inside Syria at least three times in the past few months against what it believed to be anti-aircraft and advanced ground missiles destined for the group.
Foreign forces destroyed advanced Russian anti-ship missiles in Syria last week, rebels said on Tuesday - a disclosure that appeared to point to an Israeli raid. Israel has not confirmed or denied involvement.
The army has also deployed a high-tech surveillance system along the Syria front, which immediately zeroes in on any suspicious movements approaching Israeli-held territory.
"It is critical for us to know who is sitting there - if it's an Islamist jihadi or a rebel who just wants to defend his family," the military source said.
In June, the Syrian army and Hezbollah captured the strategic Syrian town Qusair from rebel forces.
Israel watched closely and last month held a military drill that simulated taking over a northern Israeli town of Safed in preparation for possible conflict.
Israeli military sources on the Israel-Lebanon border said that despite its deep involvement in Syria, Hezbollah has not loosened its grip on the border area in south Lebanon.
"Hezbollah's legitimacy in the Arab world is cracking over its involvement in Syria," said one source on the Lebanon border. "But on the other hand, if they come under a lot of pressure they could choose to ignite the border."
Israeli commanders have noticed that Hezbollah had taken down some of its flags, as well as those of Iran, that once hung proudly in the border villages, a sign it could be worried about its image.
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