Three Syrian tanks entered the demilitarized zone in the Golan Heights on Saturday, prompting Israel to complain to UN peacekeepers, a military spokesman said.
The foray would be the first such violation in 40 years and hikes concerns that violence from Syria's civil war could heat up a long-quiet frontier.
Israel's relatively low-key response of turning to the UN suggested it did not see the Syrian armour as an immediate threat.
But the entry marks the most serious spillover of Syria's turmoil at the frontier to date. Misfired Syrian shells have exploded inside Israel on several occasions and a tourist site was temporary shut after armed Syrians were spotted nearby recently.
The three tanks entered the DMZ on Saturday and Israel lodged a complaint with the peacekeepers, an Israeli military spokeswoman said, speaking on condition of anonymity in line with military protocol. She did not elaborate on what the tanks were doing.
The Israeli news site Ynet said the tanks and two armored personnel carriers drove a few kilometres away from Israeli military positions.
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Israel captured the Golan Heights from Syria in the 1967 Mideast war. It later annexed the strategic territory overlooking northern Israel in a move that is not recognized internationally. Before 1967, Syria used the highlands to shell Israeli villages and farms.
The DMZ, which is about seven kilometres at its widest and 200 metres at its narrowest, was created after the 1973 war in which Syria tried to retake the plateau.
Marco Carminjani, an official with the UN body supervising the zone, said he could not immediately confirm the entry of the tanks. But if the report is true, he said, it would be a violation of the 1974 disengagement agreement between Syria and Israel. He said it would be the first such move in the zone since the accord.
There was no immediate comment from Syria.
Israel and Syria have been bitter enemies for decades and have fought several wars but the border has been mostly quiet for years.
There is concern in Israel that if the regime of Syrian President Bashar Assad is toppled, the country could fall into the hands of Islamic extremists or descend into sectarian warfare, destabilizing the region.
Israeli officials have also expressed concern that the frontier region could turn into a lawless area like Egypt's Sinai desert, where Islamic militants have gained strength since the ouster last year of Egyptian President Hosni Mubarak.
Rebels launch big offensive
Syrian rebels launched a dawn assault Saturday on a strategic airbase in the north of the country, trying to disrupt strikes by warplanes and helicopters that pound rebel-held towns and give the regime of President Bashar Assad a major edge in the civil war.
The assault, reported by activists, comes a day before the start of a key international conference in Qatar at which the United States and its allies aim to reorganize the opposition's political leadership and unite their ranks. The leadership-in-exile has been widely seen as ineffective and out of touch with rebel fighters on the ground..
Rebel forces attacked the Taftanaz airbase early Saturday morning in fighting with government forces that continued into the afternoon, the anti-regime activist Syrian Observatory for Human Rights said.
Joining Syrian rebels in the attack were fighters from Jabhat al-Nusra, an al-Qaeda-inspired Islamic militant group made up of foreign jihadis, according to the Observatory. Al-Nusra fighters, who are considered among the most experienced and disciplined among the opposition forces, have led attacks on other airbases in the north in past months.
The Taftanaz base mainly houses military helicopters, near the main highway between the capital Damascus and the northern city of Aleppo, where rebels and the military have been battling for control for months.
Activists say more than 36,000 people have been killed in Syria since the anti-government uprising began in March 2011.