Israel and Lebanon exchanged fire at the border near the northern town of Rosh Hanikra Sunday night. According to the IDF, the incident began after a Lebanese soldier, possibly acting alone, apparently fired into Israel at an Israeli vehicle.

Army Radio described the incident as “grave.” It said the gunman, apparently a member of the Lebanese Armed Forces, opened fire at the border fence, and that Israeli forces returned fire.

The IDF did not release any details about injuries in the incident, and an IDF spokesperson said the incident was being investigated.

The IDF was searching the area for a terrorist who may have crossed the border, and was treating the shooting as a terror incident, but ultimately no cross-border infiltration was discovered.

The United Nation’s Interim Force in Lebanon urged restraint by both sides, and the IDF said Israel conveyed a severe protest to the UN over “this outrageous breach of Israel’s sovereignty” and beefed up its presence along the border.

A Hezbollah-affiliated website reported that the Lebanese terror group put its fighters in southern Lebanon on high alert after the incident.

According to Hezbollah’s al-Manar news agency, an IDF patrol crossed into Lebanese territory and was fired upon by the Lebanese Armed Forces. It said the IDF fired flares over the border into Lebanon, and that a Lebanese soldier went missing during the clashes.

According to Lebanese state media, Israeli planes entered Lebanon’s airspace around noon on Sunday. Al-Manar news quoted Lebanese military officials saying Israeli planes entered Lebanon around 8 p.m.

The incident happened near the same spot where a bomb blew up an army jeep, injuring four soldiers, in August.

In 2010, Lebanese snipers shot at Israeli soldiers on the border, killing one and injuring another, and sparking an international incident. Three Lebanese soldiers and a journalist were killed in Israeli retaliatory strikes.

Israel and Hezbollah fought a three-week war in 2006, but the border with Lebanon has remained mostly quiet since.