Several rockets pounded northern Israel Thursday morning, causing several light injuries and Israeli forces fought back shortly by firing mortars across the border, Israeli sources said.

Dense smoke rises from Gaza city after Israeli bombardment on Jan.8, 2009. (Xinhua Photo)

Explosions were heard in northern Israel, caused probably by rockets from neighboring Lebanon and several people were lightly wounded, Israeli TV reported.

It was still unclear whether the rockets were fired by Hezbollah or by Palestinian organizations in Lebanon acting in solidarity with Hamas, which denied Thursday that it was responsible for firing rockets into northern Israel from south Lebanon.

Residents of the western Galilee bordering Lebanon were quoted as saying that at least three rockets hit their communities, with one falling close to the town of Nahariya.

Israeli police have ordered the residents there to remain close to the fortified areas, and school has been closed in the region.

In a quick response, Israel fired mortars across the border shortly after the rocket attack, both Israeli and Lebanese security sources said.

An Israeli military spokesman said Israeli mortars were targeted at "the source of fire". Five artillery shells were reportedly fired by Israeli side.

A Lebanese security source confirmed the news.

The Israel Defense Forces (IDF) said it is investigating these incidents. Assessments by the IDF Northern Command indicated that the barrage was likely to have been fired by Palestinian groups, though it could not be ruled out whether Hezbollah might have told them to fire, The Jerusalem Post reported.

During the Gaza offensive, the Israeli army and police have also been on high alert in the north, amid fears that the Lebanese Hezbollah movement might launch attacks.

Israel has warned the group that should it ignite a second front, Israel would retaliate massively.

On Wednesday, Hezbollah leader Hassan Nasrallah said that the IDF could not destroy Hamas and that his group would crush any assault Israel might carry out against it.

The rocket attack from southern Lebanon was the first since the Jewish state launched on Dec. 27 its offensive against Palestinian militants in the Hamas-ruled Gaza Strip.

Weeks earlier, several rockets were discovered in southern Lebanon aimed at Israel and ready to fire, prompting UN peacekeepers and Lebanese troops to step up patrols along the border with Israel.

The UN Interim Force in Lebanon (UNIFIL) and the Lebanese army "have deployed additional troops and intensified patrols and security control of the area," local news service Ynet quoted UNIFIL spokeswoman Yasmina Bouziane as saying.

The UNIFIL commander is in contact with senior Lebanese and Israeli military officials, she added.

Israel and the Hezbollah guerrillas went into a month-long war in 2006, which left over a thousand people dead and a million displaced.