BAGHDAD, Aug. 23 — The number of Iraqis fleeing their homes has soared since the American troop increase began in February, according to data from two humanitarian groups, accelerating the partition of the country into sectarian enclaves.

Despite some evidence that the troop buildup has improved security in certain areas, sectarian violence continues and American-led operations have brought new fighting, driving fearful Iraqis from their homes at much higher rates than before the tens of thousands of additional troops arrived, the studies show.

The data track what are known as internally displaced Iraqis: those who have been driven from their neighborhoods and seek refuge elsewhere in the country rather than fleeing across the border. The effect of this vast migration is to drain religiously mixed areas in the center of Iraq, sending Shiite refugees toward the overwhelmingly Shiite areas to the south and Sunnis toward majority Sunni regions to the west and north.

Though most displaced Iraqis say they would like to return, there is little prospect of their doing so. One Sunni Arab who had been driven out of the Baghdad neighborhood of southern Dora by Shiite snipers said she doubted that her family would ever return, buildup or no buildup.