JERUSALEM - Israeli forces on Monday afternoon fired into Syria after another Syrian mortar shell exploded outside an Israeli military outpost in the Golan Heights.

The shell hit an open area in the vicinity of the outpost in the central Golan Heights, causing no injuries or damages, a military source told Xinhua.

"In response, Israel Defense Forces soldiers fired tank shells towards the source of the fire, and confirmed directly hitting a mobile Syrian artillery battery," the source said.

The incident came a day after a stray Syrian shell landed near the same outpost, Tel Azeka, eliciting Israeli forces to directly engage the Syrian military with warning artillery fire for the first time since 1973.

According to unconfirmed reports, Israeli troops retaliated by firing a single Tamuz-type missile towards a Syrian artillery canon, prompting UN Secretary-General Ban Ki-Moon to call on both sides to show "utmost restraint and refrain from firing of any kind across the cease-fire line."

Following Monday's incident, Israel has lodged a complaint with the UN forces in the area, stating that fire from Syria into Israel will not be tolerated, said a military statement.

Sunday's shell hit meters from a previous Syrian salvo of three mortar shells last week that fell within a civilian community, one of which landed on a fence near the demilitarized zone, but did not explode. The other two exploded in open areas, causing no casualties.

Defense officials here believe that both Sunday and Monday's mortar shells were related to the inner-fighting in Syria, a by- product of its civil war, which has been going on since March last year.

On Sunday, Israeli Prime Minister Benjamin Netanyahu said Israel was closely monitoring events along the northern border and was prepared for all developments.

In recent weeks, shells as well as tanks and stray bullets from Syria kept flying across the border with Israel.

Last Wednesday, four stray bullets hit an Israeli military outpost at the center of the Golan Heights, and no damages or injuries were reported, while a military vehicle was hit by stray bullets fired from within Syrian territory last Tuesday.

On Saturday, three Syrian tanks entered the buffer zone in the Golan Heights, prompting Israel to issue an official complaint to the UN.

The UN forces in the region have increased patrols along the border to prevent any friction between the two countries.

Israel captured the Golan Heights from Syria in the 1967 Middle East War and annexed it later, an act not recognized by the international community. The demilitarized zone was created after the 1973 war in which Syria tried to retake the strategic plateau, and Syria has long called for Israel's full withdrawal from the area and set it as a condition for making peace with Israel.