Tel Aviv/Beirut  - Unknown militants fired at least three Katyusha rockets into northern Israel early Thursday morning, prompting the Israeli army to return fire into south Lebanon, Israeli and Lebanese officials said.

An Israeli police spokesman said the Lebanese salvo lightly injured two people in the coastal city of Nahariya, 10 kilometres south of the border with Lebanon. Others were treated for shock.

One Katyusha hit the roof of a retirement home in the city, blowing a hole in the roof and shattering windows.

Lebanese officials said the Israeli military fired artillery in retaliation at an area close to the southern Lebanese village of Marwaheen, located near the Lebanese-Israeli border. There were no immediate reports of casualties.

Lebanon's premier Fouad Seniora condemned the rocket attacks and the Israeli response, saying the attackers were trying to undermine stability at the Lebanese border with Israel.

There was no immediate claim of responsibility from any Lebanese militant group, and Hezbollah, which fought a bitter war with Israel in the summer of 2006, denied any involvement.

"Hezbollah informed the Lebanese government that the group did not fire any rockets on northern Israel," a source in the Shiite guerrilla group said, said without elaborating.

Hezbollah's Labour Minister in the Lebanese cabinet Mohammad Fneish said Thursday his group was unaware of the Katyusha firing from southern Lebanon.

"When Hezbollah does something, it announces it and has no problem doing so," he told reporters before entering an emergency cabinet session.

Similarly, a spokesman for Hamas in Lebanon denied that his group, which is fighting the Israelis in the Gaza Strip, was behind the rocket salvo.

"We do not know who fired the rockets, but it is not Hamas," Rafaat Morra said.

Anwar Raja, spokesman for the Damascus-based Popular Front for the Liberation of Palestine-General Command (PFLP-GC) speaking from the Syrian capital declined to confirm or deny that his faction was responsibility for rocket-launching on northern Israel.

He added that Israel "did not have the right to inquire about the source of rockets".

The Lebanese army, in a statement, said, "an unknown party launched a number of rockets" toward what it called "the Occupied Palestinian Territories." The statement added: "Israel retaliated and launched artillery shells on the southern (area) of Naqoura."

Anti-Syrian Christian Lebanese Forces leader Samir Geagea urged the Lebanese government to assume its responsibilities and take a "decisive stance" on the rocket attack on northern Israel.

He said any Lebanese faction which wanted to open a second front with Israel, which is currently battling Palestinian militants in the Gaza Strip, "should propose the issue to the Lebanese government."

"The government is the only side entitled to open a new front in the south," he said, adding that "any new front that would be opened with Israel is not in the interest of Palestinians."

The Lebanese army said its units were cooperating with United Nations peacekeeping troops in the region (UNIFIL) and were taking adequate measures to protect civilians and maintain stability.

UNIFIL Commander Maj. Gen. Claudio Graziano called for self- restraint on both sides of the border, while the peacekeeping force was placed on full alert.

Lebanese villagers in Tair Herfa said they heard early Thursday the sound of explosions at the edge of their village.

Schools in south Lebanon were ordered to be closed in anticipation of Israeli retaliation, while residents in northern Israel were also ordered to open bomb shelters and be on the alert.

A Lebanese government source told Deutsche Presse-Agentur dpa that the Lebanese cabinet was working on finding and naming the group behind the rocket attacks. The same source ruled out that Hezbollah was behind the attack, because the rockets were not "the type Hezbollah used during the July 2006," and were of an older type.

Shortly after the rockets landed, Israeli warplanes intensified flights over the western sector in southern Lebanon. At least six warplanes flew at low altitude over areas from southern Lebanon, reaching the outskirts of the capital.

The Katyushas striking Israel were the first rockets to have been fired at northern Israel since Israel began an offensive against Palestinian militants in the Gaza Strip on December 27, and the first from Lebanon since June 17, 2007 when a salvo slammed into the northern Israeli city Kiryat Shmona causing minor damage and no injuries. (dpa)