A bomb struck an oil pipeline Thursday in Iraq's southern city of Basra where Iraqi security forces have been clashing with Shiite militia fighters, an oil official said, the second such attack this week.

Despite the turbulence, Iraq's oil minister assured international oil companies that ongoing clashes would not affect oil exports and drilling operations.

"The security situation in Basra is still unstable ... but this has not reflected negatively on works at oil output and export installations," Hussein al-Shahristani told the U.S.-funded Radio Sawa.

Al-Shahristani said his ministry has more than 12,000 police "who are on high alert and deployed around oil fields and pipelines."

The bomb exploded underneath the Zubair-1 pipeline that sends crude oil from the Basra Zubair oil field to tanks for Iraq's two exporting terminals on the Gulf: al-Umaiya and Basra, according to an official in Basra.

The official, who spoke on condition of anonymity because he was not authorized to release the information, said the effect of Thursday's 10 a.m. blast could have an impact on Iraq's oil exports but the extent was uncertain.

It was the second pipeline bombing this week. On Tuesday night, a bomb damaged a domestic oil pipeline that links the Noor oil field in the southern Maysan province to the refinery in Basra, officials said. It was expected to take several days to repair the damage.

Iraq's oil exports have recently witnessed upbeat levels since the U.S.-led invasion in 2003. An average of 1.54 million barrels were sent each day through Basra in February, according to the Oil Ministry.

Basra, 340 miles southeast of Baghdad, is Iraq's second-largest city and accounts for most of Iraq's oil exports.

Iraq's average production for February was 2.4 million barrels per day. Exports averaged 1.93 million barrels per day during that month.

The war-torn country relies heavily on oil exports for revenue for rebuilding efforts and has sustained the flow from Basra even as insurgents targeted the country's oil installations elsewhere.

Oil output in southeastern Iraq is around 2 million barrels per day, according to oil ministry figures, accounting about 80 percent of Iraq's total output.

The huge portion of oil output comes from Basra Rumaila South and North oil fields that produce around 1.3 million barrels per day.

The city also is home to one of Iraq's three largest oil refineries, the Shuaiba refinery which has a capacity of 160,000 barrels a day but has been functioning below capacity at about 100,000 barrels per day.