IDF tank units fired directly at a Syrian military target for the first time on Monday, scoring what was said to be a direct hit on a mobile artillery battery in Syria that had just launched a mortar shell into the Golan.

The mortar shell hit the border area inside Israeli territory near Tel Hazeka in the Golan Heights. The IDF estimated that at least two Syrian soldiers were injured by Israeli fire, Channel 10 reported.

Israeli media reports indicated that the targets hit by Israeli tanks were Soviet-made D-30 howitzer pieces. Israeli officials said the vehicle was believed to belong to the Syrian government.

Israeli military officials had previously contended that the intermittent mortar fire was spillover from intense fighting near the frontier between Syrian President Bashar Assad’s army and rebel forces trying to oust him, and not an overt attempt to hit the Jewish state. On Monday, however, a senior Israeli official said Israel is starting to question that assessment.

“We thought it was spillover, but today we’re not sure,” said the official, speaking on condition of anonymity because the issue is still being debated among intelligence commanders.

The Syrian Observatory for Human Rights reported that Syrian rebels and government forces battled in the DMZ village of Bir Ajam on Monday afternoon just before the reported Israeli retaliation. According to the report, Bir Ajam was hit by mortar shells and there were casualties on both sides.

Clashes between government forces and Syrian rebels continued into the late afternoon, and three rebel fighters were killed, according to the British-based opposition Syrian Observatory.

On Sunday the Israel Defense Forces fired an anti-tank missile close to Syrian forces as a warning, after a mortar bomb exploded near an IDF position in the same area.

The mortar fire from Syria is widely seen in Israel as constituting errant shells rather than deliberate attacks, but concern is mounting at the potential for casualties. A shell last week fell inside an Israeli moshav, but failed to explode; residents said the consequences could have been disastrous had it done so.

The exchanges of fire, which have so far caused no injuries, are the first between the two countries since the end of the 1973 Yom Kippur War.

“We have no interest in getting in between the rebels and the Syrian army, but rather to defend the Golan Heights from stray fire,” IDF spokesman Brig. Gen. Yoav Mordechai said on Sunday.

In addition Sunday, Israel conveyed a message to Syria that it would respond to any further spillover of fighting into Israel with return fire.

Israel also filed an official complaint with the UN observer forces stationed along the demilitarized zone between Israel and Syria, and warned against further fire from inside Syria.

UN Secretary-General Ban Ki-moon confirmed in a statement from his New York office that the shelling was reported in the UN-monitored zone between Israel and Syria, but that no injuries to civilians or UN personnel were reported.

Ban called “for the utmost restraint” and urged Syria and Israel to uphold their ceasefire agreement and halt any exchange of fire.

On Sunday morning, Prime Minister Benjamin Netanyahu said Israel was “closely monitoring what is happening on our border with Syria and… we are ready for any development.”

Last week, three mortar shells from Syria fell in and around Moshav Alonei Habashan in the Golan, but caused no damage or injuries.

The IDF has been kept on high alert since November 3, when three Syrian tanks strayed into the demilitarized zone separating the two borders, leading Israel to lodge an official complaint with UN peacekeepers stationed in the DMZ.

Israel has long feared Syrian conflict spillover into the Golan. Last Monday, an army jeep was hit by a stray bullet from Syrian territory, just days after top IDF brass toured the region.

In September a number of mortar shells fired by regime forces landed in the north of the Golan Heights, and in another incident Syrian soldiers entered the DMZ.

The Associated Press contributed to this report.