NEW : Thousands pilgrims killed and many injured in attacks Thursday . Two of Shiite pilgrims are making their way to Karbala to mark Imam Hussein . Scores of pilgrims have been killed in attacks blamed on Iraqi extremists . Shiite security forces have sealed roads around Karbala .
NEW : Two pilgrims killed and many injured in attacks Thursday . Thousands of Shiite pilgrims are making their way to Karbala to mark Arbaeen . Scores of pilgrims have been killed in attacks blamed on Sunni extremists . Iraqi security forces have sealed roads around Karbala .