FACTBOX: Armed groups operating in south Lebanon

(Reuters) - At least three rockets fired from Lebanon hit northern Israel Thursday in an attack seen as linked to Israel's offensive against Hamas Islamists in the Gaza Strip.

There has been no claim of responsibility for the rocket attack for which there are several suspects. The rockets were fired from the Hezbollah stronghold of southern Lebanon, which is officially controlled by the Lebanese army and U.N. peacekeepers -- the U.N. Interim Force in Lebanon (UNIFIL). South Lebanon was the scene of a ferocious 34-day war between Israel and the Lebanese guerrillas in 2006.

Here are some facts about the possible suspects behind the rocket attack:

HEZBOLLAH

The powerful political and military group has thousands of well-trained fighters who stood their ground against an Israeli onslaught in 2006. The group enjoys strong support from Iran and Syria and is listed as a terrorist organization by the United States.

Hezbollah is the dominant military force in Lebanon, stronger than any other faction or the Lebanese army. Its arsenal, estimated to be tens of thousands of rockets, sophisticated anti-tank weaponry and possibly anti-aircraft batteries is mostly supplied by Iran via Syria.

Hezbollah still maintains a stronghold in the south even after a U.N. peacekeeping force was beefed up to 13,500 along the Lebanese-Israeli border following the end of the 2006 war. The group also has heavily fortified positions in the eastern Bekaa Valley.

PALESTINIAN GROUPS

Lebanon is home to 400,000 Palestinian refugees dispersed in 12 refugee camps across the country, the largest being the Ain al-Hilweh refugee camp in the south. These camps are home to several Palestinian groups.

The Syrian-backed Popular Front for the Liberation of Palestine-General Command has bases outside these refugee camps and in the eastern Bekaa Valley. It has Katyusha rockets as well as light weapons. The group has in the past three decades launched attacks against Israel from Lebanon.

In the most spectacular attack in the late 1980s a PFLP-GC guerrilla flew into northern Israel using a hangglider and killed six soldiers near an army base before being shot dead.

The leader of the PFLP-GC, Ahmed Jibril, has threatened to open a new front on Israel in the midst of the Gaza offensive which began on December 27.

ISLAMIST MILITANTS

The Palestinian refugee camps are also a haven for al Qaeda-inspired Sunni militants, especially in the Ain al-Hilweh refugee camp which is a hotbed for Islamist militant groups and where the Lebanese army is not authorized to enter. Some of these groups include members of the Jund al-Sham, Usbat al-Ansar, and Fatah al-Islam movements. They are believed to have light weapons and some rockets.

The militants, some of whom are Arabs, are suspected to have launched attacks on UNIFIL in the past two years. The Lebanese army fought Fatah al-Islam militants at a north Lebanon refugee camp in 2007 but some of these militants are still hiding in Ain al-Hilweh.

(Writing by Yara Bayoumy; Editing by Louise Ireland)