Iraqi officials have suggested privately that Iran, which is overwhelmingly Shiite Muslim, is backing its Shiite brethren, who form a slight majority in Iraq. One Iraqi official said more than 100 volunteer fighters have entered this year from Iran into southern Iraq, where Iran may be trying to use its influence within the dominant Shiite community there.Iran might also support extremists from the rival Sunni branch of Islam — such as Al Qaeda or the group loyal to Jordanian terrorist Abu Musab al-Zarqawi — to gain influence in the Sunni community, which is powerful in central Iraq, and to destabilize U.S. efforts to control the country, some analysts say.