A new National Intelligence Estimate on Iraq cites significant security improvements but concludes that security remains fragile, several American government officials said.

Even as officials described problems with the planning and performance of the Iraqi forces during the Basra operation, signs emerged Wednesday that tensions with Moktada al-Sadr, the radical cleric who leads the Mahdi Army militia, could flare up again. Mr. Sadr, who asked his followers to stop fighting on Sunday, called Thursday for a million Iraqis to march to the Shiite holy city of Najaf next week to protest what he called the American occupation. He also issued a veiled threat against Mr. Maliki’s forces, whom he accused of violating the terms of an agreement with the Iraqi government to stand down.

Estimates by Iraqi military officials of the number of officers who refused to fight during the Basra operation varied from several dozen to more than 100. But three officials said that among those who had been relieved of duty for refusing to fight were Col. Rahim Jabbar and Lt. Col. Shakir Khalaf, the commander and deputy commander of an entire brigade affiliated with the Interior Ministry.

A senior military official in Basra asserted that some members of Colonel Khalaf’s unit fought even though he did not. Asked why he believed Colonel Khalaf did not fight, the official said that the colonel did not believe the Iraqi security forces would be able to protect him against threats to his life that he had received for his involvement in the assault.

“If he fights today, he might be killed later,” the official said.

The senior American military official said the number of officers was “less than a couple dozen at most,” but conceded that the figure could rise as the performance of senior officers was assessed.