An Israeli tank struck a Syrian army vehicle Monday after a mortar shell landed on Israeli-held territory, the military said, in the first direct confrontation between the countries since the Syrian uprising broke out, sharpening fears that Israel could be drawn into the civil war next door.

Israel has steadfastly tried to avoid getting sucked into the Syrian conflict, but it has grown increasingly worried after a series of mortar shells have struck territory in the Israeli-controlled Golan Heights in recent days. On Sunday, Israel fired a "warning shot" into Syria in response to the shelling.

The conflict has already spilled over into several of Syria's other neighbors -- whether in direct violence or in the flood of refugees fleeing Syria's bloodshed. On Monday, a Syrian fighter jet bombed a rebel-held area hugging the border with Turkey three times, killing more than a dozen people and bringing a protest from the Turkish government.

Potential Israeli involvement in Syria could be far more explosive. Open hostilities between Israel and Syria could have wide-ranging consequences, dragging in Lebanon's Hezbollah guerrilla group and perhaps Hamas militants in the Gaza Strip on Israel's southern flank.

Israeli military officials say they believe the mortar fire landing in the Golan is not an overt attempt to hit the Jewish state and is likely accidental as Syrian President Bashar Assad's army battle rebel forces trying to oust him.

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But Israeli officials have begun to question that assessment and are now exploring whether any of the cross-border fire has been intentional. Israeli officials have long feared that the embattled Assad might try to draw Israel into the fighting in an act of desperation.

"We are closely monitoring what is happening and will respond appropriately. We will not allow our borders to be violated or our citizens to be fired upon," Prime Minister Benjamin Netanyahu said Monday in a speech to foreign ambassadors.

In a statement, the military said Israeli tanks targeted the "source of fire" in Syria after the mortar shell landed in an open area of the Golan Heights. It confirmed "direct hits" on the targets.

Israeli military officials, speaking on condition of anonymity under army guidelines, said an army vehicle carrying "Syrian mobile artillery" was hit. There was no immediate word on casualties on the Syrian side, but Israeli officials said the vehicle was believed to belong to the Syrian government.

The incident began when Syrian military units were shelling gunmen in the Syrian village of Bariqa, which lies only several hundred meters from the frontier with the Israeli-held Golan. An Associated Press photographer on the Golan side saw gunmen, presumably rebels, running as explosions shook the village from the shelling by Syrian army mobile artillery visible about a mile away.

The rebels fired back with automatic weapons, then fled, running toward the Golan border and taking refuge under some trees. A few minutes later, the rebels made their way back to the village. Bursts of artillery fire could be heard every few minutes, and about a half hour later, the Syrian shell struck the Golan, making a loud whistling sound before impact.

The Syrian Observatory for Human Rights, a Britain-based anti-Assad activist group that relies on a network of activists on the ground in Syria, said three rebel fighters were killed Monday in clashes with the Syrian army in Bir Ajam, a village neighboring Bariqa.

Rami Abdul-Rahman, the head of the Britain-based group, said rebels belonging to an Islamist group attacked several Syrian military checkpoints and that government forces fighting back for control of the area.

The state-run news agency SANA has not commented on the fighting in the area or the clash with Israel.

A number of mortar shells have landed in the Golan over the past week, and early this month, Syrian tanks accidentally crossed into a buffer zone along the frontier of the Golan for the first time in nearly 40 years. Israel captured the Golan, a strategic plateau, from Syria in the 1967 Mideast war and subsequently annexed it.

Israel responded for the first time on Sunday, firing what it called a "warning shot" into Syria after another mortar shell strayed across the frontier and landed near an Israeli military post. Israel also warned of a tougher response if the attacks persisted.

Israel has little love for Assad, who has provided refuge and support to Israel's bitterest enemies through the years. But the Syrian leader and his father before him have kept the frontier quiet for nearly four decades, providing a rare source of stability in the volatile region.

The Golan frictions were a potent reminder of how easily the Syrian civil war -- already spilling across borders with Turkey, Lebanon and Jordan -- could explode into a wider regional conflagration.

The Syrian airstrikes on Ras al-Ayn hiked tensions on the country's northern border, with Turkey.

The heavy strikes killed more than a dozen people in the town and nearly 70 wounded were brought into Turkey for treatment, a Turkish official said on condition of anonymity because he was not authorized to brief the media.

Regime forces and rebels have been battling for days over the town, which lies practically adjacent to the border.

Last week Syrian rebels overran three security compounds in Ras al-Ayn and wrestled control of the town, located in Syria's predominantly Kurdish, oil-producing northeastern province of al-Hasaka. A surge of 11,000 more Syrians escaped into Turkey on Friday following the fighting at Ras al-Ayn.

Turkish Foreign Minister Ahmet Davutoglu, speaking to reporters in Rome, said Ankara had formally protested the bombings close to the border, saying the attacks were endangering Turkey's security, state-run TRT television reported. He said Turkey had also reported the incident to NATO allies and to the United Nations Security Council.

The Syrian jet did not infringe Turkey's border, he said, adding that Turkey would have responded if it had. He did not elaborate.

In recent months, shells fired from Syria have landed on Turkish territory, prompting Turkey's military to retaliate in kind.