Special report: Tension escalates in Iraq     

    BASRA, Iraq, March 27 (Xinhua) -- Gunmen blew up an oil pipeline in Zubair area west of Basra on Thursday morning, a local security source said.

    "A powerful explosion rocked the town of Zubair, 20 km west of Basra, in the morning when unknown gunmen blew up a major oil pipeline," the source told Xinhua on condition of anonymity.

    A huge fire was ignited by the blast while a column of black smoke rose over the area, the source said.

    Fire fighters rushed to the area to put out the fire, while Iraqi security forces cordoned off the area to secure the scene, he said.

    An Iraqi oil ministry source told Xinhua that the chaos and battles of Basra did not affect the oil production or the export levels.

    "Till now, as far as I know there is no direct effect on the oil export levels," the source said.

    "Our ministry has more than 10,000 policemen tasked to protect the oil pipelines and the other oil installations," the source added.

    Earlier, officials from the Mahdi Army militia, loyal to Shiite religious leader Muqtada al-Sadr, had threatened to set the oil wells on fire if the Iraqi security forces did not halt its operations against Sadr followers in the city.

    Fierce clashes raged between Shiite militias and U.S.-backed Iraqi security forces in the country's southern oil hub of Basra, 550 km south of Baghdad.

    Iraqi security forces launched a massive offensive, dubbed "Operation Cavalry Assault," aimed at restoring order in the city where instability was spreading.