Prime Minister Binyamin Netanyahu reacted Monday to Syrian shells that landed in the Golan Heights, asserting that Israel "will not allow anyone to breach our borders or to fire on our citizens."Israel is monitoring the situation along the Syrian border closely and will react accordingly, Netanyahu said through a spokesman.fired into Syrian territory for the second time in as many days in response to Syrian shells that landed on the Israeli side of the border.Israel on Monday Strategic Affairs Minister Moshe Ya'alon said earlier Monday, before the second shell hit Israel, that Syrian President Bashar Assad is being very careful regarding the border with Israel. Ya'alon noted, however, that that there is very heavy fighting taking place in the Golan Heights as well as in Damascus, referring to Syria's ongoing civil war. The Syrian president, he assessed, has no interest in opening a new front with Israel.UN Secretary-General Ban Ki-moon on Sunday called for restraint on behalf of Israel and Syria, after IDF ground forces fired a warning shot at the Syrian military for the first time since the 1973 Yom Kippur War.Ban's office said “the Secretary-General is deeply concerned by the potential for escalation. He calls for the utmost restraint and urges Syria and Israel to uphold the Disengagement Agreement, respect their mutual obligations, and halt firing of any kind across the ceasefire line.”The shells hit as Israel suffered a barrage of missiles from Gaza, putting the IDF in the position of monitoring enemy fire along both the northern and southern borders.“In the midst of Syrian infighting, a mortar shell fired by the Syrian Army struck near an [IDF] outpost at Tel Azeka,” IDF spokesman Brig.- Gen. Yoav Mordechai said Sunday.The shell failed to cause injuries or damage. It was one of a series that hit Israeli territory.“In light of the policy instituted by IDF chief of Staff Lt.- Gen. Benny Gantz, a warning round was fired back into Syria. We don’t believe it caused injuries or damages,” Mordechai added.The IDF fired an advanced Tapuz-type missile at a Syrian artillery cannon, military sources said.“We will not accept any firing into our territory,” he added.“This was a signal to the Syrians, that we will not be so forgiving of everything that lands in a territory.”The source stressed that as of now, Israel and Syria were not in a situation of conflict.The IDF is on standby in case of security deterioration on both the northern front and on the border with Gaza.Defense Minister Ehud Barak told Army Radio Sunday, “The message has certainly been relayed. To tell you confidently that no shell will fall? I cannot. If a shell falls, we will respond.”Israel also lodged a complaint with the United Nations Disengagement Observer Force (UNDOF) on Sunday.Syria has been in the midst of a brutal civil war for over a year, and the IDF has been instructed to prevent the battles from spilling over into Israeli territory.Strategic Affairs Minister Moshe Ya’alon told Arutz 7 Sunday that Israel has sent very clear messages to Syria not to let the violence spill over into Israel.The most recent events on the Golan came a week after three Syrian tanks entered the demilitarized zone Saturday afternoon, and remained there for several hours into the evening.The tanks, which were involved in heavy clashes with Syrian rebels, encroached the decades-old cease-fire line Turkey has retaliated against errant mortar shells and violent spillover from Syrian infighting in recent weeks as well.Tovah Lazaroff and Yaakov Lappin contributed to this report.