Days of bloodshed between religious sects in Iraq show that the United States still has work to do to achieve a new, broadly representative Iraqi government, a top State Department official said Friday.

Sectarian attacks and reprisal killings that began with the bombing of a revered Shiite mosque are troubling, but do not necessarily portend further violence or civil war, James Jeffrey, senior adviser for Iraq to Secretary of State Condoleezza Rice, said in an Associated Press interview.

"It indicates that the path to national reconciliation and the path to a national compact that we're striving so much for has a ways to go," Jeffrey said. "It means we better continue working and work harder on it."

Jeffrey said that Al Qaeda leader Abu Musab al-Zarqawi is the "likely suspect" in the mosque bombing, although he said there is no clear evidence of that. He added that although neighboring Iran is trying to increase its political pull among Iraq's factions, "we see no specific line that leads you directly to Iran in any of what happened in the last week and a half."

Earlier Friday, the top U.S. military commander in Iraq said the situation has improved but conceded a major new terrorist attack would threaten stability anew.

"Now, it appears that the crisis has passed," Gen. George Casey said in a briefing from Baghdad with reporters at the Pentagon. "But we all should be clear: Iraqis remain under threat of terrorist attack by those who will stop at nothing to undermine the formation of the constitutionally elected government."