FACTBOX: Key facts on Basra, Iraq's second city

(Reuters) - Heavy fighting erupted on Tuesday in the southern oil city of Basra where Iraqi security forces launched a major operation at dawn against powerful militias, military officials and witnesses said.

Here are some details about Basra:

* GEOGRAPHY:

-- Basra is the main port of Iraq and situated on the western bank of the Shatt al-Arab, the waterway formed by the union of the Tigris and Euphrates rivers. Basra is 550 km (340 miles) south of the Iraqi capital, Baghdad.

* SOME HISTORY:

-- Basra was founded as a military encampment by the second caliph, Umar I, in AD 638 about 8 miles from the modern town of Az-Zubayr. The first architecturally significant mosque in Islam was constructed there in 665.

-- By the 14th century, neglect and the Mongol invasions left little of the original Basra standing, and by the turn of the 16th century it was relocated at the site of the ancient Al-Ubullah, a few miles upstream.

-- In the 17th and 18th centuries, English, Dutch, and Portuguese traders were established there, and Basra developed considerably during the 19th century as a trans-shipment point for river traffic to Baghdad. In 1914 the construction of a modern harbor began at Basra, which previously had had no wharves.

* WORLD WAR TO GULF WAR:

-- During World War One, the British occupied Basra. Under the ensuing British mandate many improvements were made and both the town and port grew in importance. In 1930, the port installations were transferred from British to Iraqi ownership. During World War Two the Allies sent supplies to their Soviet allies through Basra.

-- Basra's oil refinery was seriously damaged in the opening months of the Iran-Iraq War and many of its buildings were destroyed by artillery bombardments as the Iranians advanced to within less than 6 miles in 1987.

-- The city again suffered extensive damage in 1991 during the Gulf War and in subsequent fighting between rebel factions and government troops.

* OIL:

-- Oil production and exports from Basra were unaffected by the latest heavy fighting. Iraq's oil ministry said that crude oil shipments from the region totaled 1.54 million barrels per day in February.

-- Historically, two-thirds of Iraq's oil output came from southern fields and flowed through Basra. Barrels of oil shipped through Basra provide the bulk of the central government's revenue.

-- The refining facility in Basra has a 150,000-bpd capacity located near the port but it lacks independent power generation and wastewater treatment.