Lebanon's Prime Minister Fouad Siniora condemned the rocket attacks on Israel from his country Thursday and said an investigation has been launched into the incident, as tensions heightened on the northern border. "Lebanon denounces and condemns the firing of rockets and the retaliatory action and believes that such action is in violation of UN Security Council Resolution 1701," Siniora said in a statement, according to Lebanese press reports. "We have asked the competent authorities in cooperation with the UN Interim Force in Lebanon (UNIFIL) to investigate," he said. No group has claimed responsibility for the rocket firing, which lightly wounded two elderly residents of a retirement home in Nahariya. Israel briefly shelled southern Lebanon in retaliation, which Siniora also criticized. According to the IDF Northern Command, the salvo into Israel was probably fired by Palestinian terror groups and not by Hizbullah, but the possibility that Hizbullah had instructed some other group to fire at Israel could not be ruled out. Hizbullah, Hamas and Fatah all denied responsibility for the incident. Lebanese Information Minister Tarek Mitri told AFP that Hizbullah had assured the cabinet that it "remains committed to stability and Resolution 1701," which brought an end to the Second Lebanon War. A Damascus-based leader of the Popular Front for the Liberation of Palestine-General Command, Ahmed Jibril, threatened Saturday to open up new military fronts against Israel if the conflict in Gaza were to escalate. UNIFIL, which controls southern Lebanon, said Thursday that it was taking immediate measures to identify the perpetrators of these attacks. "UNIFIL is investigating the circumstances in close cooperation with the Lebanese Armed Forces," UNIFIL spokesman Andrea Tenenti told The Jerusalem Post. "The UNIFIL Force Commander [Claudio Graziano] is maintaining close contact with the Lebanese authorities and the Israeli authorities and has called for maximum restraint to prevent any escalation of the situation." In addition, UNIFIL has intensified its foot and vehicle patrols, the the Lebanese army has deployed additional units to the South in an effort to prevent any further incidents, he said. "We have a huge presence in the South," Tenenti said. "We are not 100 percent everywhere all the time. We did our best, as in the past, and in the last two years the area has been stable." He said that there are no Palestinian refugee camps in the area from where the rockets were fired, which is mostly populated by Shi'ite villages. Siniora said the rocket firing was harmful to the country's security, and liable to drag the country into an undesirable situation "that would serve Israeli designs, rather than serving Lebanese, Palestinian or Arab interests." On Wednesday, Hizbullah chief Hassan Nasrallah warned Israel in a fiery speech via satellite television "that all the possibilities are open" against Israel. "If you decided to launch an attack against Lebanon, you would realize that the July war [Second Lebanon War] would be like a picnic compared to our response," he said. He said Israel will not be able to destroy Hamas or Hizbullah. In a message to Prime Minister Ehud Olmert, he added: "I tell Olmert, the loser, the disappointed and defeated in Lebanon, 'You will not be able to eradicate Hamas and you will not be able to eradicate Hizbullah. Nasrallah also chastised "Arab leaders" for trying to mediate a truce between Hamas and Israel, instead of siding with the embattled Palestinians in the Gaza Strip. The comment was an apparent swipe at Egypt, which on Tuesday, together with France, made proposals to end the fighting. Also on Thursday, Lebanese Army chief Samir Geagea called on the cabinet to conduct an investigation and find those responsible for firing the rockets. "Opening any new front in Lebanon would not be beneficial to the situation in Gaza, " Geagea said, according to the NOW Lebanon news site. He warned that no one should underestimate Israel's ability to fight on two fronts at the same time. Yaakov Katz and AP contributed to this report.