By Eli Ashkenazi, Amos Harel, and Yoav Stern, Haaretz Correspondents and Agencies
"The state, through all its security services, will spare no effort to find the party behind this act which aims at destabilizing Lebanon," Siniora said in a statement.
Two Katyusha rockets hit the northern town of Kiryat Shmona on Sunday afternoon, in the first rocket strikes on northern Israel since the end of the Second Lebanon War last summer.
"Two Katyusha rockets landed in Kiryat Shmona, and caused damage to a vehicle and roads. There were no injuries," a police spokesman said.
A third rocket struck next to a United Nations Interim Force in Lebanon (UNIFIL) base in the southern Lebanese village of Houla.
It was not immediately clear who was responsible for the fire, but a Lebanese security source said suspected Palestinian gunmen - not Hezbollah - had carried out the attack.
Hezbollah issued an official denial of responsibility for the rocket fire. "Hezbollah denies being involved in any operation to launch missiles today against occupied Palestine," said a flash script on Hezbollah-run Al-Manar television.
The Popular Front for the Liberation of Palestine-General Command also denied Sunday any involvement in the rocket attacks.
An Israeli official accompanying Prime Minister Ehud Olmert on a visit to the United States confirmed Palestinians were apparently behind the rocket fire, adding that Israel would not retaliate.
"The prime minister was updated about the Katyusha rockets immediately after the event and has consulted with the defense minister and foreign minister," the official said. "It seems that it was Palestinians, not Hezbollah."
The official said Israel viewed the attack with "deep concern" and believed the Palestinians were trying to trigger an Israeli military response. But, the official added: "Israel will not be drawn in."
Foreign Minister Tzipi Livni said Sunday that Israel expects the Lebanese government to take control of its territories, and ensure that neither Hezbollah nor Palestinian groups can attack Israel from within its borders.
She added that Israel expects the UNIFIL to act according to its directive. "We expect UNIFIL troops to exert every effort to prevent the repetition of this kind of attack," she said.
She discussed the incident with her counterparts by telephone.
Spanish Foreign Minister Miguel Moratinos contacted her immediately after the event, to assure her that Spanish UNIFIL troops stationed in the area from which the rockets were fired were doing everything in their power to thwart any attack on Israel.
Livni is maintaining close contact with Olmert and has conferred with him about the incident.
She will also raise the issue at a European Union foreign ministers' conference in Brussels this week.
Following the attack, Defense Minister Amir Peretz spoke on the phone with Olmert and Israel Defense Forces Chief of Staff Gabi Ashkenazi. Peretz also met with senior defense and military officials in Tel Aviv for a previously-scheduled security consultation.
The official Lebanese news agency said Lebanese army troops arrived at the site from which the rockets were launched, near Taibe - which is west of Kiryat Shmona and north of Kibbutz Misgav Am. The troops found four rocket launchers with timing devices, including one that failed to fire and was dismantled by the military.
The Lebanese army also set up roadblocks in an effort to locate a black Honda that was seen fleeing the scene of the rocket fire.
Yasmina Bouziane, a deputy spokeswoman for UNIFIL, said: "Today's rocket attack from south Lebanon is considered a serious violation of Security Council Resolution 1701 and serious breach of cessation of hostilities agreement."
She added that UNIFIL is urging all parties to excercize maximum restraint.
According to police, an initial examination determined that the rockets had a diameter of 107 mm.
Defense officials instructed the Kiryat Shmona municipality to continue with life as usual, and not order residents into bomb shelters. The municipality's emergency hotline, however, will operate around the clock.
Magen David Adom emergency medical services Director-General Eli Bin placed emergency medical teams on high alert throughout northern Israel.
Kiryat Shmona Mayor Haim Barbivai called for a tough response from both Israel and the Lebanese government. "Heaven help us if we have another summer like the last one. That would be a tragedy," he told Channel 2 television.
Kiryat Shmona was hard-hit during the 34-day Second Lebanon War, during which Hezbollah fired some 4,000 Katyusha rockets at towns and cities across northern Israel.