Tension is growing along Israel's northern border with Syria, for 40 years the quietest of the Jewish state's frontiers, amid indications jihadist rebels are infiltrating the U.N.-monitored Golan Heights.

The northern boundary is the cease-fire line across the heights established after the 1973 Middle East war that divides the Golan, with Israeli forces occupying two-thirds of 3,000-foot-high strategic volcanic plateau.

This de facto border has been the least troublesome for Israel even though Syria had long been its most implacable Arab foe.

This was largely because Damascus was able to use its proxies in Lebanon, initially Palestinians and later Hezbollah, to maintain military pressure on Israel without endangering its own position.

The recent abduction -- and release -- of 21 U.N. personnel from the Philippines by Islamist militants underlined the deterioration in security.

If that worsens, it could result in the 1,250-man U.N. Disengagement and Observer Force being withdrawn. Small contingents from Canada and Japan have been discreetly pulled out.

Croatia says it's planning to withdraw its 100-person contingent after it was reported Saudi Arabia was funneling arms bought from Croatia to rebel forces.

Analyst Geoffrey Aronson, writing on the Al Monitor website, reported, "Israel has made a formal request that UNDOF remain in place with its agreed complement of forces, but the government is said to be pessimistic about the organization's prospects in view of the anarchy that's consuming the entire country."

If U.N. personnel are removed, Israel will find itself in pretty much the same position in which it found itself in 1967, when U.N. forces were withdrawn from Egypt's Sinai Peninsula. The Egyptian military entered the zone in what Israel saw as a major threat, prompting pre-emptive strikes against Egypt that triggered the Six-Day War of June that year.

The Syrian regime has effectively lost control of its sector of the Golan following steady infiltration by Jabhat al-Nusra, who attacked a Syrian military intelligence base near the largely abandoned town of Quneitra in February.

On Sunday, Syrian rebels seized a military intelligence base on the Huaran Plain 5 miles north of the cease-fire line.

"In the coming weeks and months, the increasing flow of foreign fighters into Syria will likely see even more al-Qaida affiliates operating in the border region," observed David Schenker of the Washington Institute for Near East Policy, a U.S. think tank.

Israel's worried because it sees new threats on all its borders, anarchy emerging in Egypt and Sinai amid fears the Syrian war is spilling over into Jordan and Lebanon.

The Syrian Islamists, who by all accounts are steadily growing in strength, are primarily focused right now on bringing down Syrian President Bashar Assad's regime.

But some are now talking of pursuing their jihad to Israel and "liberating Jerusalem."

In a recent jihadist video, a rebel declared: "For 40 years, not a single gunshot has been fired on the land ... toward Israel. We will free the Golan and it will return to the free Syrian people."

Assad's forces have fired a few shells into the Israeli zone in recent weeks, possibly in error while fighting rebels.

But if the aim was to provoke Israel into military reaction and distract attention from the uprising, it didn't work. Israel lobbed a few shells back as a warning.

But its Jan. 30 airstrike inside Syria, supposedly on a convoy near the Lebanese border carrying anti-aircraft missiles to Hezbollah, was a reminder that if it sees itself threatened, particularly by Syria's chemical weapons, it will strike hard.

"There's a vacuum there now which is encouraging chaos," cautioned Israeli military analyst Eyal Zisser.

Britain's Guardian newspaper reports that Israel will seek to persuade U.S. President Barack Obama to authorize U.S. airstrikes on Syria if there's evidence Syrian missiles are handed over to Hezbollah, or at least support Israel military action.

If the Damascus regime goes, its successor will likely be dominated by Sunni hardliners, and the Jewish state can expect trouble -- at the very least an attempt to push its army, and some 40,000 Jewish settlers, off the Golan to the internationally recognized 1967 border.

However, that would leave the Galilee region under Syrian guns on the Golan, which was why the Israelis stormed it in 1967, then annexed it in 1980.