A small terrorist group was likely behind the firing of the two Katyusha rockets into northern Israel on Friday afternoon, military sources said. The rockets struck open fields near Nahariya. No injuries or damage were reported. The IDF responded by firing some 15 artillery shells toward the source of the fire near the Lebanese city of Tyre. IDF sources said the rockets were 122-mm. Katyushas, a short-range projectile known to be in the hands of Hizbullah, as well as smaller Palestinian terrorist groups that operate in southern Lebanon. Israel also reportedly called homes throughout Lebanon and warned residents that the IDF reserved the right to respond to the Friday attack. The army released a statement saying that Israel holds the Lebanese government responsible for terrorist attacks that originate in its sovereign territory. IDF sources said the incident was likely isolated and would not develop into a larger conflict. Military forces were, however, put on high alert along the northern border. A senior Lebanese military official said the rockets were fired from the town of Qlaileh, near Tyre. "The rocket attack could be connected to a number of events - the eighth anniversary of 9/11, the large IDF exercise in the North this week, or some small terrorist group's discontent with Hizbullah," a defense official explained. The attacks also could be connected to the political tension in Lebanon following Sa'ad Hariri's announcement on Thursday that he had failed to establish a unity government with Hizbullah, the official said. Following the attack, the IDF filed an official complaint with UNIFIL. Several peacekeepers crossed into Israel to examine the site where the rockets hit. The last time rockets were fired into Israel from Lebanon was in January, during Operation Cast Lead in the Gaza Strip. Then, too, the IDF assessment was that a small radical Palestinian group was behind the rockets, which it fired into Israel to show solidarity with Hamas, which was fighting the IDF in Gaza at the time. On Friday, the Beirut-based newspaper Al-Akhbar reported that the Lebanese army had arrested members of a Palestinian terrorist cell that was plotting attacks against UNIFIL positions in southern Lebanon. The cell, reportedly connected to the Fatah al-Islam terrorist group based in the Nahr al-Bared refugee camp, near Tripoli, was using "innovative tracking methods" to gather intelligence on UNIFIL forces. According to the report, UNIFIL command Maj.-Gen. Claudio Graziano ordered all personnel to raise their level of alert and to increase security around their bases.