BAGHDAD, Sept. 30 (Xinhuanet) -- New violence surges ahead of the key Oct. 15 constitution referendum in Iraq, with more than 140 people, including 13 US service members, killed in the past four days.

    Just before sunset on Thursday, three car bombs went off almost simultaneously in Balad, 90 km north of Baghdad, killing 62 people and injuring over 100 others, a local official source told Xinhua.

    The apparently coordinated attacks taking place around 6:30 p.m. (1430 GMT) targeted a bank, a busy market and another location in downtown Balad.

    "After the first explosion, lots of people gathered at the scene to see what happened when the second bomb detonated, thus causing heavy casualties," the source said.

    Dozens of women and children were among the dead. Among the wounded in the follow-up blasts were Balad's police chief Colonel Kadim Abdul Razaaq and four other policemen.

    Some of those injured were transferred to Baghdad for treatmentas Balad's hospitals could not cope with so many casualties.

    On Wednesday, five US soldiers, assigned to the 2nd Marine Division, II Marine Expeditionary Force, were killed in a blast during clashes with insurgents in Ramadi, 110 km west of Baghdad.

    It was the deadliest single attack on US forces since a roadside bomb killed 14 US Marines in the town of Haditha on Aug. 3.

    The Ramadi blast brought the death toll of US troops in Iraq to1,934 since the war began in March 2003.

    In capital Baghdad, two coordinated roadside bombs struck a US military convoy on Thursday morning, damaging two US Humvees.

    In a separate incident, another roadside bomb detonated at about 8:50 a.m. (0450 GMT) near al-Shaab Stadium in eastern Baghdad when a US military patrol was passing by, damaging a US military vehicle and wounding an Iraqi civilian.

    The attacks were part of a new round of violence ahead of the Oct. 15 referendum on a new constitution.

    US commander in Iraq Gen. George Casey told Congress on Thursday that the process of withdrawing US troops from Iraq depends greatly on the results of the referendum and elections that are due to take place afterward if the constitution passes.

    "The next 75 days are going to be critical for what happens," he told the Senate Armed Services Committee, hinting that plans tocut troop numbers next year may be thwarted if things go awry in the constitutional referendum.

    Iraqis will vote on a draft constitution on Oct. 15 and, if they endorse it, a new government will be formed on Dec. 15 under the timetable of the political process.

    Sunni Arab leaders are calling on their followers to vote "no" on the constitution, which they claim was drawn up by the Shiites and Kurds who dominate the government.

    Though Sunni Arabs make up only 20 percent of Iraq's population,they can torpedo the constitution because of a loophole in the voting rules. If two thirds of voters in any three of Iraq's 18 provinces vote "no," the referendum fails, even if an overall majority approve. Enditem