June 1 (Reuters) - Lebanese troops attacked positions of al Qaeda-inspired militants entrenched in a Palestinian refugee camp in northern Lebanon on Friday with the heaviest shelling in 13 days of fighting.

Here are some facts about Palestinian refugees in Lebanon:

* BACKGROUND

-- The creation of Israel in 1948 prompted an exodus of Palestinians from their homes. Their descendants make up the bulk of the 4.3 million refugees cared for by the U.N. Relief and Works Agency (UNRWA), many in slum-like camps in Lebanon, Syria, Jordan, the West Bank and the Gaza Strip.

-- Palestinian guerrillas established bases in Lebanon in the late 1960s and fought in the civil war that erupted in 1975.

-- Many refugees enjoy citizenship in Jordan and full rights short of citizenship in Syria. Only Lebanon's estimated 400,000 refugees are confined to 12 camps and barred from many jobs.

* CAMPS IN LEBANON

-- Palestinian refugees make up an estimated 10 percent of Lebanon's population.

-- Several Palestinian factions compete for control of the camps in Lebanon, where they are allowed to keep weapons.

-- Ain al-Hilweh, near the southern port city of Sidon, is Lebanon's largest camp, with about 70,000 refugees. Other camps are in Beirut and the south and north.

-- In September 1982, hundreds of civilians in Beirut's Sabra and Shatila camps were massacred by Christian militiamen allowed in by Israeli troops then in control of the capital.

-- Battles have engulfed Nahr al-Bared camp, home to 40,000 refugees, in north Lebanon since May 20. At least 84 people -- 35 soldiers, 29 militants and 20 civilians -- have been killed. Fatah al-Islam, a Sunni Muslim group inspired by al Qaeda, made its base in the camp in late 2006 after breaking away from the Syrian-backed Fatah Uprising faction. It has only a few hundred fighters and scant political support among Lebanese or Palestinians.

-- More than 25,000 of Nahr al-Bared's 40,000 Palestinians have fled to the smaller Beddawi camp nearby.

* WHAT'S AT STAKE

-- While uprooting Fatah al-Islam is an aim most Lebanese parties would support, any storming of Nahr al-Bared might kill many civilians and would breach an Arab accord which has barred the security forces from Lebanon's refugee camps since 1969.

-- The idea of accepting Palestinian refugees permanently is taboo in Lebanon, which fears upsetting its own delicate sectarian balance. Most of the refugees are Sunnis.

Sources: Reuters/ United Nations/UNRWA: http://www.un.org/unrwa/refugees/lebanon.html

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